It's widely believed that the end of the iPhone exclusivity deal will be pretty much fatal to AT&T. The carrier of course discards these allegations, but stays very realistic. As AT&T's CEO said:
70% of AT&T customers are on family plans, which are tough to get out of. He also said that 40% of AT&T customers are on corporate discount plans, which can be a pain to let go of.
It's a very compelling point, obviously most people won't stay for the network. Just in case that wouldn't be enough though, AT&T has apparently just decided to raise its early termination fees on smartphones from $175 to $325 starting June 1st. This just 10 days after they rolled back their eligibility dates for the next iPhone.
I think we're starting to have a good picture of what's going on here. Apple will release one last iPhone on AT&T next month, and that will be it with AT&T's exclusivity. They in turn are trying to squeeze every last cent they can get out of this deal, by locking down the hell out of you.