Cena is the modern day equivalent of Hogan i.e. the golden goose cash cow. They aren't interested in pleasing the 20% of smart fans at the expense of the 80% that buy his merchandise. It sucks for Punk but it certainly shouldn't come as a surprise. Jobs are just part of the business especially when you're Punk's size.
I heard a rumor awhile back that was probably bogus that said Bret would come back to manage the Hart Dynasty but even though he and Vince are on good terms now the heat between he and DX is still white hot which makes that scenario pretty much impossible.