Regardless, this is all covered under pre-existing condition clauses and contract law: i.e. they most definitely WON'T be out the $200k from your hypothetical.
PS: Heath insurance company's are vary rational, if the risks where equivalent they would charge close to the same rates due to market forces. Instead identical group plans even for small groups can be significantly cheaper.
It is unlikely that more than 1 or 2 people in a small company will have any major health issues, so if you are signing up 20 people your overall risk is lower (in your example a $10,000 per person cost).
Combined with different feelings toward risk, usually health insurance is a win win. The annoying thing about the recent "reforms" is it mandates preventative care be covered, eliminating the ability to get pure "oh crap" coverage.
This is only annoying until you realize how many people have incurred significant costs by avoiding preventative care. Why do you think insurance companies are so quick to run campaigns for healthy-living, vaccination, regular physicals, etc.? They'd much prefer to pay a few hundred dollars a year for you on a known schedule than deal with open-ended late-stage problems.
Bob takes out a policy because it seems like a good idea, and subsequently is diagnosed with Expensive Heart Disease. He holds onto his policy with dear life, prioritizing the premium payments over things like dinner if he has to.
Joe takes out a policy because it, too, seems like a good idea. Nothing happens to him, and six months later he has a change of heart when he thinks about how many shots of tequila he could be buying with the premiums instead, so he cancels his policy.
Thus sick people become ever more concentrated in the pool of individual insurance customers.