Pokemon will return to megas.
Megas are marketable and everyone liked them.
Do you know what had in common z-moves with teras? Was that both of them despite being universally accepted by the fans, those weren't marketable. There's just so much you can do with z-moves and teras.
G-max tried to emulate megas by fusing them with Z-moves but failed as well, as Kaijus aren't just as marketable as shonen transformations, at least not as much in Japan (westoids opinion doesn't matter, they are deranged)
I'm pretty sure that teras, the most lame gimmick they ever made, were just a placeholder until the return of megas in a proper form to the franchise. After all, we are still seeing megas in the spin-off games and in the anime, the people is voting with their wallets!