Zardnaar
Legend
I have two more sessions of my current game before I start. Just waiting for Forge of the Artificer.
So let's take a specific example. The PHB dragonborn was weak. Weakest species in 2014. It's been revised twice: once in Fizban's and once in 2024. The Fizban one already invalidated the PHB dragonborn, despite being "optional" because it was clearly a patch for the fact they missed on the first one. (See also: Tasha's class "options").
By your standard, Fizban's dragonborn would not be compatible with 5e because it essentially makes the phb dragonborn obsolete. No one would pick the PHB version if Fizban is available. It's both power creep (insofar as a bad option is brought up to par with most others) and it renders the older version obsolete (again, you can choose the PHB option, but why?)
I don't think anyone could rationally and in good faith argue Fizban's Guide to Dragon isn't backwards compatible with D&D 2014.
Dragonborn is niche. How many people actually used Fizbans? I own it never had a player ask to use it. You're allowed but it came out late and got power crept out.
As I said it's technically compatible on the class side. Feats and spelks not so much. Races are mostly decent as old ones get floating stays and origin feat.
It's more if you squint. Alot of feats and spelks are not compatible you have to pick one or the other to use.
5.0 monsters are super weak is another example. Technically compatible if you squint.
All the revisions I suspect will split the playerbade. 2014. 2014 post tashas and 5.5. Power creeping out older options I suspect won't go over to well.