Pixar normally does sequels only when they like the story, Cars may seem like it was Pixar selling out but i was nothing more than them liking the concept and the characters, if they trully sold out they would have made more Toy Story movies already. But that doesn't mean it's not a bad idea for them to not let some of their characters go, Resident Evil movies are still being done because Anderson likes the s***y story and characters, not because it makes much money.
Pixar is begining to drop in quality, i didn't watch Brave or Cars 2 yet, but from what i heard Brave's negative critics have been because critics prefered the father and son storylines rather than daughter and mother, which seems kind of sexist of the critics.
Cars 1 wasn't as bad as people gave it credit for, i loved how it trully felt like the roads of the USA, Cars 2 seems in the least entertaining but reviews of 38% don't give me much hope. I never really cared much for Finding Nemo in the first place, it was good but not a Pixar film i'm in a rush of watching again.
The only Pixar film i ever found a sequel could be done was Incredibles, but ironically it's the only film they won't make sequels for. I hope they stop making so many sequels and Prequels and just make more movies like Wall-E and Up, they were some of their most recent works that i like more.
In the whole i liked all of their films (i still can't say anything about Cars 2 and Brave as i didn't watch them) but i'm not interested in Sequels or Prequels, i want original works, Brad Bird also need to go ahead and finally do his long promised Live-Action film based on the San Francisco earthquake of 1906.