Could be upsides to that, but cranking up the global food requirements (remember, a muscular body requires a lot more energy to maintain) would be a negative.

Also, what's optimal? Your body adapts to what you are doing, something we take for granted today. E.g. If you like bicycling, a permanent linebacker build may be undesirable.

It would probably be more ideal to set some kind of fitness "floor", rather than remove the mechanism completely.