Not sure what's going to happen with the sequels, but just for the record, A Bug's Life did $162 million domestically. Cars, at $241 million, is closer to Monsters Inc, which did $255 million.
But your point is valid... in the world of Hollywood expectations, Cars didn't fully deliver.
I'd hate to see them abandon original concepts in favor of sequels, however. Even just looking at it from the business side, every franchise runs its course with regard to theatrical films, and you need new material to build new franchises.
If they've got a great story to tell for MI2, fine, tell it. But just looking at the numbers for a particular film or two and using that to decide to shelve original concepts would be a long term mistake. Disney didn't exactly quit making original films, but certainly their lack of success in that area, despite whatever success they had with sequels, is what led at least in part to the need to acquire Pixar in the first place.