How would a Universal health care public option put private health insurance out of business ?

How would a Universal health care public option put private health insurance out of business ?
--------------------
If the public option was efficiently managed, was open to anyone who applied, and offered decent coverage at more affordable rates than private insurance, it could drive private insurance providers out of business because they would lose customers and the business would no longer be profitable for them. They would probably switch to offering supplemental insurance for things not covered by the public plan. That's how it works in many other countries. The limited and watered-down public option that the Democrats proposed wouldn't have been a competitive threat to private insurance companies. But they were afraid it could have been expanded into a real, "robust" public option in the future, which is why they killed it.
Source