Due to the rollout problems with the ACA website, some people are concerned that only the sickest will buy insurance. This in turn would lead to a death spiral and doom the program.
Let me offer a counter force to that scenario. Remember the scene from Rainman (here at the 1:20 mark) where Rain Man tells his brother there are lots of queens left in the blackjack deck? The deck is now a profitable bet for the player. Well, the pool of uninsured people is somewhat like a blackjack deck. If many unprofitable cards (people) are removed from the deck (pool of uninsured) that leaves more profitable cards (people). The blackjack player should double down, and so should the insurance companies.
If initially only the sickest sign up, that means the available profit per uninsured person goes up. From the view of the insurance companies, each uninsured person is now more profitable for them. This, I would think, will induce the insurance companies to go after the uninsured with all they have.
I have a real world example from a few years ago. I had to take care of changing my mother's medicare provider. I decided on a medicare advantage program. The competition was so fierce, because the profit was so good, that insurance agents came to the house to sign my mother up.
I think there is a good chance that the competition between the insurance companies for the most profitable people will prevent an ACA death spiral. Maybe it won't all be through the ACA website, but they will have a lot of incentive to get healthy people signed up. Some way or another, they will be crawling over each other to sign up the healthiest.