I can't remember what the rest of the posts here say, so if it's already mentioned, sorry!
I just had an idea, thinking about the amount of evolutions in the game. Almost every Pokemon has an evolution - seemingly a lot more than previous ones, or is given that impression. Even ones that aren't necessary have evolutions. This could be a sign that they were designed not to get future evolutions, seeing how the general public related to Sinnoh's mass evolutions of older Pokemon. Due to this, older Pokemon won't get any more cross-generation evolutions, to please the fans and give GF more of a reason to be trusted (I don't know the word, but so it'll always be the same, like they won't go back on the idea they came up with, with future designs that make a certain Pokemon worse?)
Alternatively, in future games, there will probably be ways to breed Mamanbou and Afro Bull (which I forgot the name of), with Luvdic and Tauros respectively, allowing them to be alternate evolutions. And thus, those will be the last Pokemon to ever recieve evolutions from a previous generation. All in all, that idea is less likely than the former one, because based on what we know, it's probably the way GF is going about it.