I wonder the same thing, but some how a similar system seems to be working in Germany which is cheaper than what we spend.
I think the idea is that you can have your own private insurance if you still believe it better than the public system. The other thing is that they expect the cost of insurance to also go down making it cheaper for employers to offer insurance. Of course that wouldn't happen right away. But I'm sure plenty of companies would keep their private insurance soley based on the fact that they are against a public option or it is something they offer and no one takes advantage of it.
Instance - Mammoth mountain offers a private insurance option which costs $7 a week. It is the worst insurance plan in existance and there for only about 10% of the workers are on the plan. If the public option became available, it is likely it would be cheaper for them to keep this plan than pay taxes and everyone who doesn't want it would just go on a public option.
I believe in germany the only private insurance is purchased by the individual in which case these are wealthy people and they do get treated before people on the public plan... but everyone does get treated.