I can't remember that a Gengar deck ever was BDIF in the DP era. Yes, there were a lot of archtypes with Gengar, most of them came down to the SF Gengar which indeed is overpowered because of the unpredictable Poké-Power. Cursegar is good, but in no way broken. LostGar, yes, it's strong with Lost World, but it doesn't seem to be unbeatable. Gengar DP and the other Gengar from AR were almost unplayable. In the ex era, Gengar ex wasn't that good - can't remember any other Gengars there.
Pidgeot broke a format. Gengar didn't.
You mean, like Dialga which can stop trainers and PokéBodies or Palkia which can snipe around and send Pokémon to the Lost Zone? Oh wait... Uxie, Azelf and Mesprit are legends, too. Guess they also had a little more influence to the format than Gengar ever will have.
If we get RR-on rotation, you'll see your Arceus soon enough on the battlefield.