See, that's where I think there will be a difference between "we are consolidating spells into 3-4 major lists" vs "we are scrapping the spell system and using ADEU powers" style changes. The game will continue to innovate as both tastes change, and things get better refined. I think of it more akin to how Basic, 1e and 2e are compatible because the major mechanics are the same or similar.
What I don't see changing in 6e would be things like proficiency bonus, the six-saves based on ability scores, the class/species/background character design paradigm, spell slots and upcasting, advantage/disadvantage, stuff like that. But I could see a version where subclasses no longer align to the levels of 14/24 for example. General compatibility due to running on the same engine, but not carrying if things line up 1:1 like 2024 was.