Mortality possibly defines plenty of our behaviour and development: while people usually take the default assumption that we'd go in a positive direction without that constraint, I do not think it's a given.
Just like we can't really predict weather (as another complex system) too far ahead, we can't really predict how something this significant changes brain development — IMHO at least.
Say the lifespan doesn't become infinity, but rather 10x ~ 800years. How do you imagine things to change? It would certainly mean that people can take up much more ambitious projects instead of the usual ~30 year constraint.
I do share your view that positive direction is not a given, but what evidence do we have that it would be worse than right now. Maybe we should be cautious of the risks.
I am simply saying that we don't have evidence either way, though if development of medical knowledge is any proof, we mostly keep learning how much we don't know (if you've ever faced any uncommon and thus unusual medical predicament, you'd know what I mean).
Heck, so much money has gone into preventing hair loss, and there does not seem to be a simple answer to that either ;)