A very simple question about the 'public option'

No one I have ever met who is eligible for Medicare would dream of turning down its coverage.

And therefore the "public option" would be so terrible because.... ???
Medicare is of course a "public option" in spades. I remember the debates before its enactment in the 1960s, about how the coming of "socialized medicine" would be the end of the American way.

Of course now we have a system that is taken for granted as a central part of the American way. Yes, yes, I am aware of the arguments (as laid out here, here, and elsewhere) about the distortions and cost pressures within Medicare. Still: as a matter of politics I have always thought that the route toward health-coverage reform in America would be steady expansion of the eligibility standards for Medicare. First down to age 60, then 55, then...

I know that "logic" tells us only so much about health policy debates. But, seriously, how can people with a sound mind and a straight face take Medicare as part of the landscape but consider the "public option" an abomination? Just curious -- but genuinely curious.