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.

you have to take into account your brain specialization as well, an as such, there are already people who are capable to think of something others can't, there's only one problem with them, since they are too focused on "being smart", they have practically none social capabilities, and are refused by society as total idiots. So it's only a mater of time before either they are recognized, or human's brain develops enough to allow them to gain some social capabilities without getting more "stupid", but then there would again be another humans who would turn that social capabilities to be "smarter"