They said that "when everyone's super, no one will be", so presumably it's along the lines of "if everything is awesome, then there will be nothing to compare it to as the new level of everyone being good at everything makes being really good at something the base standard, so no one is awesome because they'll all be boring and average".
And he probably doesn't have anything against non-powered superheroes, it's just that it was a semi-unintentional riff on the Fantastic Four. With the Parr's standing in for the members of thta team (except Dash), and with Syndrome in the role of Dr Doom, being a guy with no inbuilt superpowers (other than magic, but that probably would have made it too obvious).