Quoted By:
They'll be gradually weeded out of the generations.
So by Gen 10 we'll only have a couple. And then they'll be relegated to cool events. There most likely will not be anymore "true" mega evolutions except maybe starter pokemon and legendary "primal" evolutions; as in; legendary pokemon will always get cool new forms in remakes that are technically mega evolutions, but they'll just be called something else since even Gamefreak acknowledges that Mega Evolutions were a huge fucking mistake.