I dunno, we haven't even passed the first playtest. I don't mean to single you out—I think everyone is jumping the gun at this point and that the discussion over whether this is a non-edition, 5.5e, 6e, New Coke, Breakin' 2: Electric Boogaloo, Gundam ZZ, or Shin Godzilla is unproductive, divisive, and utterly annoying. We're just not a point where that discussion is relevant or meaningful yet.
I don't mean to be annoying certainly. I'll try and keep that in mind though as I post about the topic.
I think a lot of us are just drawing on experience.
Having watched the change from 1e to 2e, which was initially sold as just enacting the common house rules of 1e tables (and I think it genuinely was that at first) to seeing it turn into a new edition.
Then seeing the "conversion documents" to alter 2e characters into 3e characters only to realize rapidly that was really not something which could work nearly as easily as suggested.
Then the 3e to 3.5e change, which was again sold as backwards compatible when it really wasn't, devastating hundreds of third party publishers whose content was no longer really compatible with the new half-edition of the game.
Then the change from 3.5 to Pathfinder for many, which was sold as 3.75 by many at the time and which rapidly became it's own game where Pathfinder players, who had intended to keep using many 3.5 books, almost always ended up just ditching those 3.5 books after a while and just using Pathfinder books. PF2e was similarly initially sold as backwards compatible, but those claims I think were dropped pretty soon. And of course Pathfinder isn't WOTC, but the basis of the game is the basis of WOTC's original game, and the designers mostly came from WOTC, and I think it's fair to look at their transitions and language used for those transitions for this topic as somewhat informative for how these things go as well.
While 4e was not sold as just an update to 3.5e, it also had a soft half-edition with Essentials. I personally didn't see that change as drastic as a half-edition, but it sure looked like either a loud minority or a majority (depending on your perspective) did view it as a very drastic change and clearly in the realm of a half-edition. For sure, many PCs did have to change to keep up with those rules.
5e was again not sold as just an update to 4e (or 4.5e), but the language they're using for this playtest is nearly identical to the language they used for pretty much every single other prior "update" transition.
Claims of backwards compatibility just have never once held up under scrutiny for these things. And with this first playtest doing exactly what prior such changes did - already causing issues with backwards compatibility with prior supplements to the game, it really takes an awful lot of optimism to believe it will end as truly backwards compatible. And I am a very optimistic person.
In a lot of ways I am looking forward to OneD&D. I like a lot of these changes. I just don't expect to be able to use a lot of my supplement books after the transition. I think they will be kept in mind with hope initially, and we will start with just the new books "to test them out" with the intent of adding back in the old stuff after 3-6 months, and we will end up never picking up those 5e supplements again as they collect dust on our shelves. Because that's how it's gone with all these prior transitions for us. And I think for a lot of groups.