1. Myostatin inhibitors are already in development. We're already using a drug that stops us from getting fat, why would we not use a drug that prevents atrophy?
2. This is entirely focused on what would be efficient at a global scale, while decisions are made on individual's desires. People (in general) want to look muscular and fit; it's as hardcoded into our reproductive desires as anything else is. Given increasing resources, is it reasonable to assert that people will choose to totally forgo their biological body? Why is it impossible for them to have the same productive advantages while retaining a physical body for when they want it?
Human desire trumps production, even in the long term, or at least medium term. I could eat monkey chow every day and never have to do dishes or cook ever again, and save an extra 10 hours a day. I could wear the same thing every day so a machine can fold it. I can put my brain in a jar to avoid commuting. But no matter how much technology advances, if I still spend 8 hours working with my brain implant or whatever then I'm still doing at worst 24% as good as the guy working 24/7. Why would it ever be worth giving up such basic human pleasures as eating or sex just for 4x the salary?