X/Y remakes are probably way in the future, though, and while Mega Evolution might return there (especially if they go with a "faithful remaster" approach like they did with BD/SP), returning in a remake is not really my point. Nor was my point that Mega Evolution is definitely and absolutely gone (the first two words of that sentence are "I think", after all), but that I wouldn't be so sure of it ever returning as a staple mechanic in anything other than a potential remaster of X/Y, and certainly wouldn't be sure of new Mega Evolutions ever being added.
I'm not really sure what new standard evolutions have to do with this; I wasn't one of the ones who apparently thought that new standard evolutions wouldn't happen despite the fact that they already happened with Sw/Sh. Nor would I be so confident of Megas returning simply because of their popularity or design or whatever. Megas were quite controversial when they debuted, and while much of the fanbase has warmed up to them over time, they aren't quite as ubiquitous as some people want them to be. As for designs, that's sort of a non-point as well; they apparently insist on recreating models and animations from scratch for each new game anyways, so if anything dropping Megas frees up development time. It's not like there aren't thousands of other assets that serve their purpose in a game or two and then are discarded in future games.