A point against there being important, chunky undiscovered insights in to intelligence is that if there were such insights, they'd likely be simple, and if they'd be simple, they likely would have been discovered already. So the fact that no one has yet discovered any such brilliant, simple idea is evidence against them existing.

We can't consider "chunky undiscovered insights in to intelligence" - and then argue that they don't exist because they would have already been discovered. We can't have already discovered "undiscovered insights" - and we have certainly discovered plenty of big insights already. The problem is not so much that big insights don't exist, it is more that we seem to have discovered a lot of them already.