Would the Public Option End Private Insurance Companies?

From the Times article D. sent:

"Every Humana employee interviewed, including Mr. McCallister, predicted that a public plan would place private insurers at an impossible disadvantage, without duplicating their efficiencies.

"Humana’s profit margin was 2.2 percent in 2008 on revenues of nearly $29 billion. Its revenues have more than doubled since 2004, with almost all of the growth coming from the sale of privately administered Medicare Advantage plans. Those plans now account for the vast majority of Humana’s business, a real vulnerability if Mr. Obama succeeds in cutting Medicare Advantage because of its comparatively high administrative costs."