Dunno if this was heavily acknowledged, but only Gengar and Starmie had stayed OU for all generations. But with Mega Gengar on the listing, Gengar may leave OU not by Demotion, but by promotion to ubers. Quite the case, dont you think?
The mere threat of mega Gengar makes normal Gengar a lot better. Nearly every Gengar switch in is met with a switch from the opponent because they think you will have shadow tag next turn. Which means a free sub for Gengar.