You’re not looking to insure against the chance you get asthma: You have asthma, and you’re looking for someone healthy to help pay for it. If too few healthy people sign up for Obamacare relative to the sick (and remember, all else equal, they have to be charged the same regardless of their health) those healthy folks will get an increasingly bad deal as their premiums are covering the costs of more and more sick enrollees and more and more healthy folk drop out of the exchanges. That’s the “death spiral”.