I think you're right. Every time a new edition is announced -- even the relatively short WotC editions -- for every fan questioning "What's wrong with
this edition? Wouldn't it be better if TSR/WotC committed to supporting one evergreen edition?", there's a fan saying "Thank Jeebus; the current edition is really starting to show its age! I'm tired of all the bloat!"
As each edition goes from fresh-off-the-press to 5+ years old, a growing portion of its fans become disillusioned as a result of their growing understanding of the rules. It's not something that I can personally relate to, but there is a population of fans who can be relied upon to buy a new edition because they've "Seen behind the curtain" of the current one, and are looking for the sense of wonder and magic that comes with a new edition.
Then there are the fans who will buy at least the core books of every new edition just to "support the hobby." This doesn't counter any arguments that long-term support is good for fans in general, but from WotC's perspective, it's a good argument in favor of frequent edition changes.
Then there's the fact that the more editions D&D has, the more variety it has; which I think is a wonderful thing. Whatever your favorite playstyle and ruleset is, there's a D&Der or a potential D&Der who wants something different. In fact, the only way that D&D is ever going to come close to dominating the ttrpg market isn't with One Edition to Rule Them All -- it'll be by having An Edition for Every Taste.
(Even then, there'll always be a market for other ttrpgs, but you get the idea.

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So as much as long-term support may sound like a great idea, and heck, a lot of fans might even agree, I think history has shown that it's unimportant -- and arguably undesirable.