>>58016771
I think logically they should get to stay in the main series permanently because they were "new" Pokemon. Mega Banette was a Banette evolution, like the counterpart to Dusknoir. If Banette is in the game then it should also be able to mega too. Something like Dusknoir gets to keep it's final evo but Banette doesn't feels wrong. These designs were new Pokemon besides dex number.