The sales/success argument of current entries doesn't hold much water either... again, MKWii and Brawl were tremendously successful, and Nintendo simply didn't port them to the Wii U; -- Heck, similarly, we had full backwards compatibility if you are into the past entries.
Vanilla MK8 and Smash4 were much less succcessful games, of course (because of the platform itself), but they truly evolved their formulas and made better games in many aspects.
As much the point of roster cuts are so controversial these days, Brawl also proved that as long you have something "cooler" to replace it (Subspace Emissary for many), it's not that important. And again, mechanically, the game was inferior to Melee even.