Also it's possible that human intelligence already reached the most general degree of intelligence, since we can deal with every concept that could be generated, unless there are concepts that are uncompressible and require more memory and processing than our brains could support. In such case being "superintelligent" can be achieved by adding other computational tools. Our pocket calculators make us smarter, but there is no "higher truth" a calculator could let us reach.
The past decade has seen a huge number of problems widely and confidently believed to be "actual hard mode problems" turn out to be solvable by AI. This makes me skeptical that the problems today's experts think are hard aren't easily solvable too.