>> If superheroes are common <<

In Terramagne they are not common but are pretty well known. Many people with superpowers are "crickets" who hide what they can do. Others are "blue plate specials" who use their powers in ordinary jobs, not fighting supervillains. Then supervillains are the next-biggest group, and superheroes are the minority. The rate of superpowers appearing in the population is increasing.

>> you're going to get super-scientists and super-mages and some of them are going to develop super-healing <<

These things are all true ...

>> and then disabilities in general might be a thing of the past. <<

... but this is not. No matter how advanced your science or magic, there will always be some things it can't fix. Dr. Infanta talks about this in "Lifeyears."

Consider that superpowers create new disabilities as well as having the potential to fix old ones. Subwoofer's canine features create a terrible social handicap, and he got those (along with his superpower) from getting caught in supervillain/superhero crossfire.

>> Or a cosmetic thing where your replacement limbs look weird. <<

For an example of this, see "Dr. Doohickey and the Problem of Locomotion" and "Dr. Doohickey and the Mad Science Scrambler."