We don't have this right now, that's the point. Yes, there are super-high-end prosthetics available to the uber-rich, and there is the regular stuff. But even the "regular stuff" is out of reach of most Americans, especially the millions that are uninsured (or underinsured by the employer) and thus SOL.
The USA currently has the full range of medical technology commensurate with how much you can afford - but the problem is that much of the really critical care (surgeries, etc) is completely out of reach of the average individual. It truly is a system where the rich receive the absolute best and the poor (or even middle class) can just crawl into a corner and die.
"Besides, http://web.mit.edu/first/segway/ is one of many <$1000, DIY Segway projects, so this tech is getting cheaper and more accessible."
This only proves my point. DIY geek projects aside, there is no low-cost Segway competitor on the market, likely due to patent issues (I have not looked into this myself, so correct me if I'm wrong). This is the same situation for pharmaceuticals - there is no incentive to reduce drug costs when your patients must buy from you and only you, due to patent law. So yeah, there are a bunch of DIY solutions, but still no competition on the open market, and thus prices stay up.
I don't see this as an either-or situation; don't we have both right now? Especially if you look at legs instead of arms.
"... his own highest-profile project; the Segway hasn't gotten significantly cheaper or more accessible despite years of R&D,"
That doesn't make any sense at all. The Segway is the cheaper, more-accessible version of the IBOT, a mobility system. Besides, http://web.mit.edu/first/segway/ is one of many <$1000, DIY Segway projects, so this tech is getting cheaper and more accessible.