I agree with the rest, but this is off the mark. NPC classes made it absolutely trivial to make those NPCs even in seconds just by slapping a few skills & abilities on whatever creature you want. On top of that the NPC classes allowed a trivial explaination for why their version of a thing might not be the same as the one PCs (could) get.Without 3E we would still be THAC0. Without 5E we would still spend hours to create NPCs that the party kills in seconds.