+1. Thanks for this feedback, I've edited as follows:

"If perfect rationality doesn't exist in at least some circumstances(edited), then if you want to assume it, you have to prove that a relevant class of problems has a perfectly rational agent."

The point I was making is that you can prove for a large class of real world problems that such a rational agent exists, so most things remain the same, except for the fact that you have to add an additional line in your proofs quoting a general result that proves a perfectly rational agent exists in the particular type of situation.