To put it really simply, if you don't need a profit, and if you are only as efficient as your competitors, you will drive them out of business if you are not constrained in some fashion from doing so. (Capital is the usual fashion to wipe out competitors, since non profits have trouble raising it. In the health care context, arranging it so the public option takes on more unhealthy people is the more likely way to do it.)
Since a real public option properly created to not be constrained from doing so WILL drive private insurers out of business, it will not be allowed to happen. It may be called a "public option", but it won't actually be allowed to operate as a public option should. A public option which won't destroy the insurers in time, is also a public option which can't drive down prices effectively.