The Congressional Budget Office has said in its analyses of the various bills before Congress and the US Sentate that the bills containing a government health plan would result in some people losing their private insurance. This would either be because lower-cost, high deductible plans would be legislated out of existence or coverage mandates would rule existing plans as "insuffiicient" coverage, even if the policy holders are happy with them.
A public option would also have government backing, and even if it is intended to be self-sufficient, it would merely be the latest in a long line of "self-sufficient" government-backed operations, like Amtrak, Fannie Mae and Freddie Mac, to be guaranteed a bailout if they continuously operate at a loss. Insurance companies may be able to lower costs, but they cannot compete with a company capable of operating at a loss continuously without the fear of going out of business. It is much more likely that a public "option" would overrun the market, leaving it as basically the only option left - basically, what happened with Medicare.
We could call the Bush administration fiscally responsible - that wouldn't make it so. Putting "option" on the name for a government-run health insurance company doesn't guarantee its status as one, either.