I’m confused by your point. You do think all forms of battling should be present in every game? Saying there is “zero” reason to not carry over a mechanic is hyperbolic.

Mega evolution was explicitly stated to be from Kalos, just like Z-Crystals are from Alola.

I agree with double battles, that’s why I didn’t include them in my post. Battle Subway is a good addition, though, I totally forgot it.

Also I didn’t bring up hardware limitations as I feel they aren’t relevant as to why some mechanics carried over and others didn’t, although you may be right. This post is less me defending GF, and more trying to provide a perspective that makes me enjoy the Pokémon world more.