Changing the Sunset Ranch Market into Pixar (or any other theme) is a terrible idea. It would really screw up the remaining theme of Hollywood Studios. That theme has already been ignored in places, but the whole front of the park still embraces the LA/Old Hollywood theme, and the Sunset Ranch Market is part of that -- it's basically a tiny version of the farmer's market in LA.
You can currently walk from the entrance of the park to the Tower of Terror without ever leaving that theme. Changing the Sunset Ranch Market would have you leave "LA", enter a fake Pixar area, and then go back into "LA" for the Tower of Terror.