Blue
Ravenous Bugblatter Beast of Traal
What makes you think so? After the blastback with 3.0 to 3.5, in 4e they just introduced Essentials, which was fully compatible in the way I am suggesting. What makes you guarantee that they will make revisions when they have both (a) already shown in the previous edition that is not how they do it anymore and (b) already put that to practice in Tasha's where nothing old is changed, just new options added.You're welcome. They can change things while still being fully compatible and run any previous character or adventure without much conflict. They could for exemple revise some core rules on falling, reduce a number of creatures having darkvision, change how racial ASI works etc while still being compatible.
But i am fairly guaranteed when looking back closely not every little part of it will be as fully compatible as they pretend because they will want a revision over a new edition roll out, while incorporating many new changes they want in the game. It won't just be an errata to the 3 books, it entails more than that.
In other words, please defend that they will break with the policy that they have had for both 4e and 5e to something else. We have what I am saying already established by their behavior, provide equally strong examples or discussion from WotC that they will break this behavior and do revisions as you are stating.