Eviolite's a strange scenario though. You lose consistency when you alter the types of Pokemon, which we've had in both Gen 2 and Gen 6, so consistency can be broken. But when you have new evolutions, they usually introduce it by a method unseen before for consistency, so you do have this element of some things being acceptable and some not being, so I feel like evolutionary arguments based on Eviolite could go either way. They could even combine the two, and release a new evolution method and specify Eviolite to not be compatible with evolutions obtained in this manner. That would maintain consistency.
Having new evolutions and Eviolite not being compatible would just cause confusion though.
The only option is to remove Eviolite, but they haven't really removed items since Gen 2 to 3. Gems are still in it and would still work if obtained in the game.
We'll have to see, but Eviolite is at the very least a massive barrier that has to be overcome for cross gen evolutions.