Given the glacial speed of reform in the U.S. (something like these new exchanges have been in the works since Nixon proposed a similar system), I would put that fairly far down the list of risks a small startup should worry about. A much bigger risk imo is just whether the value-add of their tools over the official state websites will be enough to gain traction.
Hard to imagine a motivated startup NOT being able to offer more value. State-run websites almost uniformly have abysmal usability, are slow, and are ugly. Sure there are exceptions but I think there will always be a lot of niches, or even bigger openings, to fill. And when you're talking about a market that is going to be compulsory for every adult, even a niche can be quite rewarding.
I have to be honest: as an IT professional, I'm extremely impressed with how well designed https://www.healthcare.gov/ is. I understand its having some problems under load, but I think we're all a bit forgiving here about traffic problems when you're receiving 5 million visitors/day in traffic.
