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>WAAH WE SHOULD HAVE GOTTEN AN EVO
Why, though.
You're asking for a new Pokemon to replace your bro, essentially. Mamoswine isn't Piloswine. Tangrowth isn't Tangela. Rhyperior sure as hell isn't Rhydon.
The point of megas is to provide both a power-up and a different look at an old Pokemon, something you can't do if you outright replaced it with something else.
Don't think I don't want cross-gen evos though, because when done right they're pretty good additions. However I just don't see why they can't really exist with megas.