Upper limit to human intelligence

Every other species has a limit to intelligence and humans are no different. So there won't ever be a quantum theory of gravity or unification of all forces. Simply because we aren't smart enough.

yes, there will.
just like totally unintelligent molecules can combine to form bacteria which form plants which form simpler animals which form humans, gradually increasing intelligence, humans can develop more intelligent beings.
Sure, I may not discover the next greatest thing, but I can make a robot that can make a robot that can make a robot of sufficient advanced thinking capability to do it for me.