That is bigger than the last two gens, but not by a lot. This decision is really irritating in that case. I could only excuse it if they had a ton of new pokemon they were adding like gen 1 or 5.
It still wouldn't have made any sense, would it? All the old Pokémon already have their models and many of their animations done, it simply makes no sense to remove them and it never did. This is strictly a Game Freak thing.