Gengar is not at all outclassed by rotom-a.
Gengar still has a monstrous special attack stat and possesses blistering speed (both stats equivalent to that of latios).
Originally Posted by Fat auramaster
Rotom-A was used 56,118 times, which puts him 15th, behind Gengar.
Rotom-a has its own uses and is very good overall. However, you can't say gengar has disappeared (or has been outclassed by rotom) while it is still sitting at spot #12, although as I understand this was a dramatic decrease.
Rotom primarilly outclassed all of the bulky ghosts; if you want an offensive sweeper most people still (as the stats prove) turn to gengar and it's 24 greater base speed and 25 greater base special attack.
Although yes, many have stated before that scizor led to the rise of bulky offense teams (and gengar's fall to "only" 12th).