Aging. Nothing brings more suffering. Resource needs are considerable due to long preclinical & clinical studies necessary, perhaps also due to novel paradigms which will have to be invented along the way.
Wouldn't it be interesting if we could witness geniuses honing their unique thoughtcraft over millennia? Just how much advanced culture are we missing due to ephemeral, short-lived, desperate nature of our best and brightest?
But I'd counter there are biochemical reasons the best work comes during such a short span.
It'll likely end up being easier to extend that neurologically peak productive period further into late middle age, than stretch an entire lifespan (which may or may not extend that period anyway).