This is, quite frankly, the single biggest issue with "one-Gen-only" special gimmicks. Mons get buffed and nerfed around the competitive metagame, which is dominated by the gimmicks, and then the next Gen they're either stronger or weaker than they should be because they carry those buffs and nerfs with them into the next Gen without any changes.

Base Gengar was already barely Ubers without a Mega Evolution...once his Mega was nerfed and Megas went away, he was left in the dumpster with no compensation. He wasn't the first mon this happened to and he won't be the last until either A) we see generational gimmicks carry into subsequent generations (but not Z-moves, let those stay dead please) or B) Game Freak stops phoning it in and actually balance the entire dex from one generation to the next (which they've definitively proven they have no intention of doing).