Look to the state of Washington to see what a health insurance death spiral looks like.

Here's an older article talking about what happened in the state of Washington. Focus on the pre-ex clause. Even with an individual mandate (which doesn't impact 47% of the population and the rest will be a very minor penalty). If we don't impose a pre-existing condition clause on a person who chooses NOT to take coverage during an "open enrollment period" then we will have the same issues as Washington did. It's not the individual mandate, it's also the pre-ex that can kill insurance.

http://www.forbes.com/sites/aroy/2012/03/30/want-to-see-a-health-insurance-death-spiral-visit-washington-state/