There's alot of people who could cure people like Ben and Banner but it would just be a horrible move for Marvel Comics so they dont do it. Here's a small list of just some of the people that could do it:
Genis can manipulate Energies and radiation in almost every way. For example, he's gotten rid of sicknesses, froze people by sucking out every bit of heat around them, sucked out their air supply, brought back the dead, healed broken bones, absorbed nukes, absorbed radiation bombs, caught things on fire by increases the heat around it, created after-images of himself, and even created a light-made duplicate of himself. I know this doesnt show he can heal Ben or Banner, but he even said in an early CM issue when he and the Hulk fought Wendigo that he could heal the Hulk if he wanted, but the Hulk, enraged, instead punched him.
yea he knew right there that he could help because his cosmic awareness told him that it was gamma radiation which he can manipulate therefore making him very able to cure Hulk but Hulk wanted nothing of it.
My logic is either A: he doesn't trust Strange's magic or my favored option B: he doesn't want to even bother with a guy like Strange, a guy who banishes demons on a daily basis, to fix him.
That's not my logic that's me trying to think like Ben and probably failing.
ben was getting healed back and forth back in the earlier days, mostly by reed's machines. i believe after the last attempt, he made the decision to return to being the thing forever for the greater good. he seems to be of good humor about it these days and likely just hasn't considered a new "cure" lately.
With some of the people like strange or reality manips, enery manips, and things of the sorts, why couldn't they just give him the ability to change at will?