As basically a lifetime musician (and second generation) I'd like to give some counter-points to your hypothesis regarding the "diminishing returns" notion and

As basically a lifetime musician (and second generation) I'd like to give some counter-points to your hypothesis regarding the "diminishing returns" notion and deliberate musical practice. I'll try to keep this light hearted - so as a guitarist, one learns the rules, and practices them, then grows and grows and grows. Eventually, one learns how to break a rule, but over time, bend it into something that resembles an existing rule or otherwise still interests an audience, and create rules whereby none existed before. Sometimes minor mistakes or oddities become wholesale innovations in the right hands (ex: Jimi Hendrix, Tom Morello) but are very much a byproduct of deliberate, routine, intense attention and practice. My experience is that music performance has an incredible power to both be-shaped-by-and-reciprocally-shape the human brain. The inherent elasticity and plasticity cross cultures. Generations. One of the most consistent observational effects though is that practice makes better in most cases.