Looking at the class changes like whats up with the Druid, do you still think it's "compatible"?
If someone shows up to your OneD&D game with a 5E Druid or Paladin or whatever, is that fine? Older classes and/or races being different and/or better....?
no... I think (much like 1e/2e and 3e/3.5) most tables will tell people "great idea, but here update with the new book"
and then we get the argument what compatible even means... to some it seems 3e,3.5,4e,and 5e are already 'compatible' since the stats are the same and the rolling method... for others things like status effect class features and other things make them VASTLY incompatible...
My bet is that what WotC wants (and everything I have heard leads me to believe this is it) is DM side compatibaility but player facing incompatability.
I bet 2024 we still see alot of cross over, maybe into 2025 (depending how late in 2024 the books come out) but over time everything 5e will just be "nah just upgrade...
and some of it HAS to upgrade. You can;t have 2 players hit with a level of exhaustion and it effect them differently. You can't have jump rules different at the same table. The same way you couldn't show up at 9 out of 10 (or more) 2e tables with a wizard with a 1e fireball with no level cap... or how 9 out of 10 tables wwould not let a 3.0 haste spell into 3.5.
Now I bet anything someone will try to "prove" me wrong by saying they played at a a table that did all of these things... but they are by far the minority I am sure.
HOWEVER I myself will disprove the strawman of "it CAN'T be done with a game in 3.5 that let players bring in any d20 characters and with 4 D&D characters we had a M&M superhero (a speedster) a Jedi from the saga star wars rules and a Jafar (not usre I spelled that right) with a p90 and staff weapon from stargate...
I ALSO ran a multiverse game that slowly got weirder and weirder... and I seeded what everyone thought was galactus (from marvel comics) a survivor of the last multiverse... until he showed up as a human wizard (keep in mind this is 5e) and he said that was a misunderstanding, he survived through multi multiverse deaths... and when someone hit him I said "Don't bother with damage, cause ping that's one off my stone skin" and my players flipped realizing I had taken a 2e wizard took him through 3e, 3.5,4e and now into 5e...
so it CAN be done. I had a whole 'order' of monks in that multiverse from 4e lead by a warlord as well.