I think we're forgetting one thing: Pixar's contract with Disney is over. Cars is the last movie in the deal, and unless things change, the studios are about to go their separate ways.
Any sequels at this point would be done entirely by Disney, without any aid from Pixar. Disney would have to do EVERYTHING - including rebuilding the 3D character models from the ground up, because Pixar still has control of those.
Worse, going from how recent Disney sequels have turned out...I'd wish for none of them. Going from the quality of the direct to video quickies, I'm afraid to see what they'd do to the Pixar characters.