That said, that's roughly what the ACA ("ObamaCare") will likely end up with, if it doesn't get derailed, except that it'll take a few years to get there because 50 states is more complicated than one.
Actually, it could scale reasonably well, but for political reasons wasn't set up that way. So instead, each state will set up its own exchange; I suspect some will be MA-quality, some will be better, some will be poorer.
Note that the reason you can apply for it right away, unlike other state "benefits", is that it's not really a benefit - there's no subsidy (at least, not for self- or startup-employed folks like me who very well can afford it), so there's no real cost to the state of letting 1, 2, or 17000 more people buy in (and in fact there's an advantage, because you increase the risk pool).