Things like strokes or cumulative CNS damage primarily affect older people, so genetic fixes to those problems would have very little evolutionary selection pressure on their side.

It's also possible that genetic/biological means of turning on CNS regeneration in higher mammals would require a complexity that has not had time to evolve yet. Just because we evolved to the point of human-level intelligence doesn't mean we are perfectly evolved by any stretch of the imagination. It means we are, in a sense, a minimum viable biological entity that can acquire our level of intelligence.

I'd caution against randomly flying across cities. There's probably a very good reason why we don't naturally do that.

Just pointing out that this is a weak argument.

The chunk I’m worried about probably can’t get much worse (it’s dead).

I remember reading an article on neuroplasticity into old age. And people with extremely high neuroplasticity had better cognitive function throughout their lives however were much more likely to have dementia in old age. It's probably similar I'd guess.

Medicine is all about doing things that we don't do naturally. If we did it naturally we wouldn't need doctors.

Depends. Could be good long term, I suppose, but might require a long hospital visit even in the case of a good outcome. Enough for evolution to prune it.