For everyone who's complaining about single stage or even 2 stage Pokemon getting Mega forms. I recognize that the concept of a Mega Pokemon is that it is a Pokemon that had reach it's evolution limit and but can temporarily break that limit and reach new heights of power for a short time. And that makes it seem like once a Pokemon get's a Mega it's "locked in" to it's current level of evolution. But one thing to keep in mind, 2 years ago No One thought that Veunsaur, Charizard, Tyranitar or anything with in it's 3rd stage was ever going to see a higher form of power. I mean they were at "the limit" which can now be broken. So here we are now. Just becasue Mega Sableye implies Sableye has no alternative evolutions or forms doesn't mean we won't see entirely new methods of evolving and increasing weaker Pokemon's power some years down the road. Mega evolutions set a new "limit" so let's look froward to seeing how Mega Sableye will break that.
That's called power creep, and nobody I know who takes any kind of strategy game seriously is fond of it.
It's perfectly reasonable to "ask" for regular evolutions along with mega evolutions, and if they just keep churning out megas like this the metagame could become over-centralized on them, and I doubt anyone would want that either.