By ending exclusivity agreements, carriers will have to compete for customers.
For example, if AT&T, Verizon, Sprint, Tmobile all offered the iPhone, they would be competing on price of service. AT&T charges $30 for data, so Verizon says "we will give you data for only $25," so people who's contracts have expired go to Verizon because it costs less. Then, Sprint says "$20 data" and they get the new business. Then Tmobile says "$19 data" and now no one is going to AT&T, so AT&T lowers their data fee to $20 to attract customers again. Eventually, they all hit a price point in which they can not go any lower and still balance their budget sheets. Maybe that amount is $18, and so all four carriers eventually end up charging $18. That is how the free market works. Without exclusivity agreements, they would be competing for texting plans also.
Right now, the major competition price war between carriers is with the "Family Plan."