But I don't remember any 1Ever folks running around, although I assume there were some.
They absolutely did exist.
Literally one of my first experiences with RPGs was being told that I'd made a serious error in buying 2E, even though 1E was no longer in print, and real AD&D players only played 1E. This was in 1989. And when I got on the pre-internet and internet in 1992/1993 there was still no shortage of people declaring 2E to be inferior to 1E, and a lot of grudging stuff about only playing 2E because they'd been forced to. That continued to at least the mid-'90s, at which point I think because people were wondering if D&D would even survive, the negativity went down a lot.
But as late as the early ENworld days we got people saying they'd gone from 1E to 3E and skipped 2E because it sucked.
3E was largely welcomed because it was perceived that it might bring an end to the D&D-is-dying-out era that TSR had presided over. A self-fulfilling prophecy in a good way for D&D. 3.5E was not received as positively as you suggest, I'd say, but most naysayers were pretty annoyed with 3E period by that point - I know I was - so tended to be moving away from D&D.
4E was pre-ruined by WotC's completely demented marketing campaign, insanely ill-advised statements from the WotC person in charge of D&D (who wasn't the lead designer), a meme that it was "basically WoW", which itself was the direct result of the extremely ill-advised statements, the incredibly dumb change from OGL to GSL (and accompanying basic lock-out of 3PPs), and completely mishandled but widely discussed attempts at a VTT. It's like if you planned a campaign to derail the launch of an edition, this is basically what you'd do. It's honestly a tribute to 4E that it did as well as it did.
5E was an apology edition, and very well-received by those it was an apology to, less well by those who it wasn't an apology to. But then it luckily caught a cultural zeitgeist in like 2016 (which had nothing to do with it being 5E, frankly, and everything to do with it being "the current edition of D&D in 2016) and now we have way more people who've never played any other edition playing than those who have, as you say.
While I'm specifically asking about the above, and thus mostly focused on 5.5 and whatever comes after that, feel free to speculate about how you think the player base might change - if you think this is another boom with an eventual contraction, or whether you think we're in an era of continual expansion. Of course with technological and global considerations, it is hard to think about where we might be in a couple decades, but at least we an speculate on the 2024 revision, and perhaps whatever comes down the pike 5-10 years after.
There will inevitably be a contraction at some point, it's just a matter of when.
But in the short-term, an edition change is potentially a smaller deal, you've illustrated it's less of an issue with product, but also the new people playing are very tech-savvy and virtually all of them have played videogames, many of them have played MMOs or MMO-like games, so they're used to new versions of things replacing the old, to things being updated/changed, and so on. They're likely less scared/angry about it than earlier generations too.
So if, as WotC keep imply, 5.5E is basically a 1E to 2E or thereabouts level of change, I don't think we're going to see much turmoil. I particularly don't think we'll see even the 1E/2E level of break, because a lot of the 1E fans clearly liked that 1E was "edgy" and weird, and roughly-made, but 5E is slick and modern, and 5.5E/6E will likely also be slick and modern, so there will be no real point of differentiation there. You'll inevitably get some grogs mad about some ridiculous nonsense, like maybe they hate Feats as an article of faith or whatever, but they're not likely to be a major deal.
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