New PHB characters can play at the same game with old PHB characters.That's what is said, but I have real doubts that it is truly "compatible", especially given the obvious differences between things. It's not like you can sub in aspects of the old book with the new.
That's enough to prevent a schism. And the eventually switch of everyone over.
That's fine. They can still play with new players, using their old adventures.And I do not think everyone is moving over because that's not how edition changes work. Doesn't matter which edition change it is, there are always a small but substantial portion that are too invested in the old edition that they stay there.
As long as the underlying numbers are the same, with both the new and old ranger doing 30 damage per round, there isn't a problem with them both being at the same table.
You could just as easily think of it as any other expansion book, providing additional races and classes. Except they have the same name as the old ones.