Apple has stuck with AT&T because they signed a long term contract to give AT&T exclusivity. They had to do that in order to get the phone out. They had approached the other carriers, and no one else was interested in carrying the phone unless the carrier got to control the design of the software. If Apple had not been willing to give exclusivity, its likely AT&T too would have passed on the iPhone.

Without iPhone, there would not be Android. Even if Google had still developed it, they would have had the same problem Apple had--phone companies not willing to let Google be in charge of the software.

It's only because Verizon and Sprint and the rest desperately needed something to compete with iPhone that they were willing to allow Android.