U of Oregon Economist on pre-existing conditions restrictions

Mark Thoma, an economist and Professor of Economics at the University of Oregon, discusses the impact of attempts by Democrats to eliminate the ability of insurance companies to screen for pre-existing conditions:

If you prohibit insurance companies from screening based on pre-existing conditions then people can just wait until they are sick to enroll which raises costs, and thus premiums for everyone.

This seems to make sense to me. Why would I buy a private policy when I’m well? I can just wait, and then buy “coverage”, which is really just buying a discount on services I already know I need.