There was a ton of discussion about this, and at the time, we heard over and over that the books would be compatible. As I suspected, this isn't really going to be true, and the expectation is that you'll adopt the new classes. And it makes sense because we're going from the perspective of "please keep buying the 2014 PHB" to "please buy the 2024 PHB."
There isn't anything wrong with that attitude, of course, because WotC wants everyone to adopt the new material. That completely makes financial sense. What I find cynically amusing is all the people who argued so vehemently that of course the old books and classes would be compatible and you could keep using them.