Doing just mysteries, minus the serialized plot, I don't think most of those segments of the fanbase (more power to them) would really care.
This is correct. To say that most people who follow DC do so for the BO plot is to project the interests of the international (specifically the Western) fanbase onto the Japanese one. It's hard to appreciate for people who live outside of Japan, but DC really is a (pop) cultural institution there, the kind of staple of television that families can sit around and watch at random without necessarily being fluent with the broader storyline (akin to Doctor Who in Britain). The movie franchise officially achieved blockbuster status with The Darkest Nightmare, and between it, the anime, the manga, the merchandise, and cultural influence, DC is almost its own industry. Everyone knows DC, even if they don't watch it or follow it, just like everyone knows about the Simpsons (to borrow your example) even if they've never watched a single episode. I don't think it's far-fetched at all to suggest that most people who watch Japan in DC do so casually, just to enjoy a good mystery. The BO aspect is the least essential part of DC's domestic success, which not coincidentally is why Gosho can get away with doing BO cases so rarely. Whether or not the BO is important to DC's international success is a different discussion, but the point stands.
doesn't care to do his best unless he feels that ok isn't enough for sales.
I think you can argue that Gosho's writing has stagnated without presuming purely monetary motives.
Plus, The Simpsons, which started much sooner and more simplistic never even promised more than clever comedy and is still on the air.
I think this alludes to an interesting point, which is that many current fan's dissatisfaction with DC, including your's, Antiyonder, stems from the fact that it has demonstrated that it can and has been more than just an episodic mystery series with fun characters. For all that I just went on about how the BO plot isn't crucial to DC's appeal back at home, I do think that stuff like the BO plot line and various developmental arcs undergone by characters does elevate DC quality-wise and entertainment-wise above many other mystery shows, and among the Western fanbase it is definitely the reason that many fans are still on board. (And it's part of the reason you see so much interesting discussion and disagreement between DC fans in particular; aside from a consensus that everything up to the end of the Vermouth arc was basically perfect or at least very good, we can't seem to agree on what makes DC still worth following, whether it's the romance, the plot, the mysteries, the character dynamics, etc.). Just look at this forum: discussion of characters and of the BO plot far, far outstrips any sustained discussion of the cases, which are, whether we like it or not, the core of this mystery series and to what Gosho dedicates the majority of his brainpower.
The incongruence of (a subset of) the fandom's interests and Gosho's focus has created this curious phenomenon where fans determine the quality a mystery series on the basis of...well, everything besides its (case) mysteries. The majority of Gosho's work is essentially ignored in favor of what other people find compelling, which would not have happened if the series hadn't attracted people not necessarily super interested in the day-to-day cases in the first place (a testament to the initial quality of DC's myriad other elements). If the intellectual puzzles compelled not just because of their intrinsic design but of how they related to the characters and the overall plot, then it makes sense that a decrease in the quality of the latter two would leave people frustrated and uninterested in the former, no matter how technically well-written. Mind you, I'm very sympathetic to that perspective, and I myself, while still loving most of DC's cases just for their intellectual value, do find them enhanced when Gosho manages to ground them in thematic statements and character moments; it deepens the emotional resonance of what might otherwise be dry puzzlebox mechanics and as such allows the cases that fall short to shine regardless (personally, part of the reason I could never get into Kindaichi despite its sometimes brilliant mysteries was its utterly bland characters and lack of alternate sources of investment). But even with that said, I do find it genuinely odd how little attention is sometimes paid to the episodic cases themselves, which are probably the strongest thing in modern DC, though like everything else there has been a decline. To deny the detective aspects of of a detective series would be to deprive yourself of a pretty major source of the series' entertainment.
But I agree that Gosho once did a better job of juggling the multiple components of his series, and when they were all in sync he could produce magic like Desperate Revival (and really the entirety of the Vermouth arc). He still can; it just occurs much more rarely now. What we're seeing really is an inevitable result of overstretched length, of DC becoming a victim of its own success. DC today is not produced by the same mind of DR, of the 12 Million Hostages, of Black Impact, heck even of Clash of Red and Black. Gosho's been worn down by age and routine. He's no longer the magician who can tell emotionally rivetingly and narratively thrilling stories while still hiding that the status quo hasn't really been altered at all; his strings are more visible and his sleight-of-hands less graceful. But I don't believe, like some might, that Gosho has become a cynical money-grabber content to produce mediocrity as long as the bucks keep rolling in. He doesn't need the money, and being mangaka is brutal work. More importantly, he's still demonstrated a willingness to do new and fun things in his cases, even in recent years--like telling the cbar case from Kogoro's perspective-- and sometimes even subverting his own tropes (the alibi-locked-room that wasn't in the coffee aroma case, the outside culprit in the Red Woman case, the lack of a culprit in the Code of Love case, etc). Those cases see Gosho exploiting the audience's familiarity with his particular tropes to mislead and surprise them. Beyond that, Gosho still writes a healthy variety of case types and works to produce different scenarios to keep things interesting. And, sometimes, Gosho still manages to craft compelling character-based dramas within the framework of a traditional mystery, as in Sera's second case.
I think the problem, aside from Gosho understandably struggling to keep up the acrobatic act that he managed for the first 15 years or so, is two-fold: routine and popularity. Gosho has been doing this for so long that I think he sometimes slips into auto-pilot. There will be times, when you have been doing something for so long, that your mind simply dulls. For us, standing outside of brutal work schedules and not immersed in the same world for 22 years of our lives, coming up with fresh possibilities and innovative turns without irrevocably damaging the status quo is simply much easier. There will be times when Gosho is not inspired, and I think there are times when he feels tired of the series; his one-shot Tell Me a Lie, his breaks to work on MK, and his fantasizing in interviews about writing historical manga indicate to me a man who sometimes needs to get out of a Conan headspace. The problem, sadly, might be that Gosho doesn't actually remember how to do that. His recent MK work has essentially been DC MK-style, despite the narrative possibilities and structural freedom that MK's particular internal logic and characters provides him. He still writes it like a traditional three-part DC story. Gosho still gets inspired sometimes, but it seems to occur more in the realm of his cases. Then there is the second problem, the one that necessitates the status quo in the first place: DC's ridiculous popularity. The status quo is necessary to preserve its accessibility to the general audiences who watch it episodically and who go to the movies. You're probably right that there are ways to introduce major changes (like Ran discovering Conan's identity) that still retain the formula while injecting it with a fresh dose of possibilities offered by the altered character dynamics, but you might be assuming that Gosho has complete creative control over his work. DC has grown way beyond just Gosho. Story decisions that he makes have an impact on things beyond just his manga, and when you're in the middle of that web radical transformations can be difficult. This wasn't such a problem in the past because, again, like you said, Gosho was once capable of producing great storytelling even while preserving the status quo.
I say this because if he was really interested in the characterization half
I think he's still plenty interested in the characterization half, he just does less of it. Super unpopular opinion around these parts, but I think Bourbon is a great character with an interesting arc and the perfect foil to Akai. Vermouth's struggle to keep the only two people she cares about out of trouble has bene highlighted and foregrounded by virtue of Bourbon's arc. Akai's transition to Okiya doubled both as mystery and a redemption arc for the character, done counter-intuitively through his domestication (really I feel this character choice on Gosho's part--for a cool FBI agent--doesn't get enough appreciation). Gosho's problem is more that he's allowed his main characters to stagnate at the cost of shifting the spotlight to other, newer people. This is, again, in part a product of the series unhealthy length. Haibara had a fantastic arc of growing from a cold, cynical, suicidal paranoiac to someone who has discovered new meaning in life and gotten a second chance at a childhood she never had, but at this point her arc's basically over and there isn't much left to do with her outside of comedy. Conan had an arc of learning humility and crafting the close bonds Shinichi didn't have, but that could only last so long and has even been somewhat undermined by some events in the Bourbon and Rum arc. Gosho wrenched a lot of great drama and comedy out of Shinichi's predicament, but even that well, one of the things that most distinguished Conan from its mystery contemporaries, has begun to run dry. Perhaps this problem of stagnation can be resolved if we'd just do away with the status quo entirely, but that brings us back to the problem of its existence to sustain the DC industry. Sometimes Gosho finds new means through which to explore his long-existing characters (like Ran and Sera's conflict in the detective agency hostage case), but they no longer occur with the frequency they once did.
All that being said, as I've expressed in the past, I do believe the status quo is slowly raveling, but so slowly it's barely perceptible. But it's definitely there. And to address your main area of focus, Antiyonder, I think the folks over at the Red Thread page have put together a persuasive argument as to how we're building towards a climactic Ran suspicion arc. But like everything else, it's paced excruciatingly slowly, at this point somewhat ludicrously (Ran's growing suspicions have been hinted at for at least a decade now). We are working to a conclusion, even if we're moving so slowly it feels like we're running in place. And I know that's a scant comfort to many, and I wish things were different. But as I said, DC has become a victim of its own success. Considering its length, I appreciate that Gosho has managed to keep it a good series for as long as he has, even if it no longer scales the heights it once did. I only hope it concludes before it goes the way of Naruto or Bleach.