I'm not convinced. Our brains are well know for their plasticity. Our musculoskeletal and circulatory systems: not so much. The brain is capable of rewiring itself after some quite traumatic injuries, of course the people who go through rehabilitation after a traumatic brain injury do so with the focus of someone whose life depends on it.

I would suggest that most people reading this are capable of Bellard level productivity, if their life depended on it.