Well, if we're talking mutants in general, we never started with "the original five". You also had Xavier, Magneto, Vanisher, Blob, Scarlet Witch & Quicksilver (say what you will...), Mastermind, Toad, Namor, Unus, ... we kinda started out with a lot.
My only problem with having so many mutants is that they only promote a handful. So we might get some great moments with a "new" (or at least, new to us) character... who then falls into obscurity for sometimes years afterward. And when you have a favorite that isn't one of the select few that the editors are okay promoting, you're left hunting for scraps...