I think the error here is assuming mind and body are separate things. In reality our body, even our gut bacteria, has a huge effect on how we think and feel. There's a reason why sensory deprivation or dissociative anesthetics like ketamine have such a huge effect on cognition. I just don't think the human mind is designed to be a brain in a jar through virtual reality or whatever. I think whatever attempts we make at doing that would probably drive a person insane.
To that I can add from personal experience, that indeed cognitive capacity and muscular health are tightly linked. I remember that on the peak of my form in a gym, I was able to code better, longer, and keep larger mental models in my head while doing it. Compared that to the period after back injury, where I was unable to do any physically challenging activities, I noticed a slump in cognitive abilities as well. So I can't see, how evolutionary, population with weaker and weaker bodies can keep producing highly capable individuals. And yes, of course there are plenty of cases such as Stephen Hawking and alike, but these probably are exceptions that prove the rule.
