I think there's a bigger point that many are missing here. The point that I see is that someone put in a lot of work and made something of incredible quality readily available to the masses. You make valid points as to why the pro version costs so much more, but that's really besides the point. The key here is that a quality prosthetic is available to the uninsured, those in a 3rd world country, to anyone who wants it. It's something that the maker community can build on and improve to create a device that really is better than a pro model. If people were paid for this, labor costs would be insurmountable. Instead, prosthetic technology is being pushed forward by the people, for the people, for no real cost to the end user. Brilliant.
And as far as not being able to use a 3D printer, come on. I can't reliably fix my car every time, but I sure as hell can find a friend with the knowledge and skills, can watch youtube for easy stuff, and can have someone pay for it. I don't have to buy a new car when the oil needs changed, and they don't have to buy one arm at the cost of an arm and a leg.