I'm all for advancing human understanding, even if it might lag behind machine understanding, but there is a way to have machines prove they understand something we don't: ask them to build something functional that we can't. If your hypothetical future comes to pass, and we ask the machine to build a cancer-curing healing ray, and it works, I'd say that's its own win. Then we can spend however long it takes trying to understand how it works. It's depressing and disempowering in a way, but I think overall I'd be happy with that future.