Once you allow in pre-existing conditions, the risk-based model goes away. It’s no longer “insurance”.
It's no longer insurance if it doesn't cover you once you get you get sick. Because, sooner or later, unless you are on MH370 or get run over by a truck, you will get sick.
So, how do you define "pre" in "pre-existing"?
Obviously, a responsible citizen should maintain insurance and be fully covered for life. And an irresponsible citizen should be left to die. Howz the polls on that? LOL!
And then there is the whole employer-provided insurance mess. Employers should not be involved in health-ensuring their employees. There shouldn't be anything special about 29 hours or 50 employees. Employees should be responsible for their own insurance, and it should be fully portable across employers, state lines, etc.
Any viable health care system will have to deal with the free-rider problem. One way or the other.