You and I have different definitions of what constitutes a simple change.To say they don't have an easy way isn't necessarily true. They could simply remove fairy type, reset typings back to gen 4 and do it that way. Obviously that kind of move wouldn't sit well with the vast majority but it would work. And would also mean less programming with the type interactions.
And whilst I'd like the return of non fairy Togekiss, I would like to see as many pokemon included as possible.
In any case, as Autobot N noted, we already know Fairy is still a type in this game.