Finding Nemo is a bonafide success, and this success drove the Subs to be remade with Nemo. Nobody is thinking that Nemo 2 won't be a success. I don't think that Disney thought that the CBJ movie would launch a franchise, and it didn't.
Nemo is a company cash cow and keeps the company afloat after disasters like John Carter and the Lone Ranger, people have kinda voted with their wallets and so Nemo kinda deserves a place in the park. After adjusting for box office gross, Nemo is Pixar #1 film of all time.