This is just steroids for muscle growth. Why not try steroids for bone strength? Chemicals to make the bones harder, the marrow denser. The brain continues to develop, even as an adult. As the brain is "exercised" it develops more folds and more neurological pathways. Who's to say we can't speed up that growth as well?
In regards to this, from an optimization standpoint... there's flat limits, as I understand it, to just what bone and muscle and nerves can
do and still be bone and muscle and nerves (and not steel and wiring or whatev'.), and it's entirely possible (though I'd have to defer to someone actually working in the related field(s) to know if it's
plausible) that you can get entirely greater returns on your investment with replacement over improvement. If we can get 10x improvement in functioning by replacing an arm with a prosthetic for a tenth or hundredth what it'd take (in terms of resource investment) to get that level of improvement via biological improvement, well...
And, as I understand, getting notably improved bones and muscles and whatnot, beyond the current human limits is a... non-trivial project. Even in relation to the barriers related to cybernetic replacement. Denser muscles, stronger bones, etc., so forth, so on... there's costs to them (caloric in particular), and infrastructure, so to speak (want denser muscles, need stronger bones, and better circulation, and who knows what else), issues related to them. We
should progress along both lines of research, of course (because there's a flat guarantee they'll be a positive feedback interaction between the fields.), but my personal guess is that cybernetics is going to be a lot more fruitful when it comes to major external stuff, as well as having certain internal stuff (re: neurology, particularly) that physiology just can't really
do, or at least not nearly as well, or as cheaply, or with as few nasty side effects.
But then again, we're a long way away from biological wi-fi, insofar as I'm aware. Maybe by the time we're capable of stuff like that, current supposition will be largely out the window.