This video from The Character Sheet is all doomy and gloomy, but that isn't why I am bringing it up. it points out that up until the new FR books, there has not been a D&D book that requires the 2024 rules.
SO, what does a big adventure campaign look like that REQUIRES the 2024 rules. Yes, the old adventures are intended to be playable with the 2024 rules, but WotC has made no claim that new adventures will backwards compatible. So what elements of 2024 can be leveraged here, and what does that adventure promise to be in 2026?
Dragon Delves is right there. It requires the new rules. So the premise of this post is just incorrect, the FR books are not the first books to rely on the 2024 rules.