As for the 50 I agree. It would be tough if the new one is only f/2, kind of a step back. And I agree that I would probably pass on it (despite how bad Canon needs a decent, consumer level 50) if it was that slow. With how good optics/designs are getting, one of last big advantages primes have over zooms is their (more often then not) faster speeds. That is why I have not understood the new 24mm and 28mm, them only being f/2.8.
But the prior non-L 24 and 28 primes were f/2.8 as well -- Canon simply modernized those prior designs.
I have the 28 f/2.8 IS and I am truly fond of it. It's such a sweet spot of IQ, features and size/weight. It's a perfect walkaround on my 5D3, IMHO. Unless you are shooting a lot of astro or environmental portraiture, I'm hard pressed to think of a need for f/2 or faster in those focal lengths.