I hear the arguments for why GF wouldn't patch X/Y because it could decentivise players from buying OR/AS.
I'm gonna do one of them hypotheticals on why they should, okay? Items, including Mega Stones, can't be traded through Pokémon Bank, right? X/Y are patched so its players, who likely haven't bought OR/AS, can fight against Pokémon like Mega Marshtomp and Primal Groudon. "Oh snap!", Jo Everyperson exclaims, as Primal Groudon glows its glow before dealing a one-hit KO to the one-time master of the Kalos arena, Mega Tyranitar, with its brand new "Precipice Blade" move. "I want to make the earth dance, too!", Jo Everyperson thinks aloud to no one in particular. "I will buy Omega Ruby and/or Alpha Sapphire tomorrow at my local video game retail outlet."
tl;dr The cost to patch X/Y for basic battle compatibility with OR/AS might be justifiable if enough profit were to be made from additional sold copies of OR/AS.