There's the saying in science that progress happens 'one funeral at a time,' but that's not because older scientists are less intelligent - it's more like they begin to intellectually ossify. Or in the arts, the critics that rejected Matisse's 'Woman With a Hat' weren't stupid, but their thinking had stagnated.

I think we underrate the value of a fresh perspective. People living longer lifespans would dilute that benefit, imo. Sidenote: this is also an underrated issue with LLMs and their application to science - collapsing to single modes of thought over time.