From a gameplay perspective, I do think there should not be any "Condition", be it a disability or a disease or an occult status, that you're stuck with it once you have it. I don't think the fact that the Zombie was permanent in Sims 2 was really popular.
From a realism standpoint, I do agree that there are disabilities that people are stuck with forever, but others are curable. The cure doesn't have to be magical, though. Training and therapy could do the trick. And for those that imagine the disability to be permanent, just don't visit any or not the correct therapy.