To add to this Commander Keen was released on a very limited platform. More people were gaming on Nintendo systems than personal computers. If Commander Keen was released on Nintendo things may have gone differently.

Even if that were true (and I don't think it is), Commander Keen should be compared to Mario 3 (which still came out over a year earlier than the first Keen), not Mario. And 4-6 are most appropriately compared to Super Mario World, which was released the same year.

Keen was/is great, but Mario 3 and Mario World are on the shortlist for best game ever.