[natded]

Intelligence has trade-offs, costs and other limitations or we would have maxed it out hard (ie. an "intelligence explosion" would have occurred).

Increased mental illness might be one, other one is the brain size being limited by the hips of mother. Brains / intelligence also has energy costs which we might not have been able to optimize for yet.

As such, I would say that intelligence is a cost that you run. It allowed for reproduction and survival for humans when they moved towards colder regions for example, but there's little to no reason to maintain that intelligence if you don't need to adapt anymore. I think you need harsh material constraints (I don't know, space faring) to keep it from shaving itself off in order to optimize the human machine.