Private insurance doesn't require '100% participation'. Social insurance does. That's why we, usually, have government perform that function - because they already have the power to force participation in the form of taxes. The only way private companies can provide public insurance is if, as you've suggested, everyone is required to buy in.
ACA is delusional at its heart. People want socialized health care but they want to pretend its not socialism. That fantasy has us diving head first into a middle ground that is truly the worst of both worlds - forced public participation fueling and private profits.
Even if there were no 'Public Insurance' as you call it. Even if we returned to no, zero, government interference, or participation (however you choose to label it) Private insurance is in deep trouble.
Billy Bob and LaQuan don't want to, and WON'T buy Health Insurance unless they're FORCED to.
So Billy Bob wraps his 85 Camaro around a Tree one night at 3:00AM and gets carried to the E-Room by Ambulance. He has no Insurance, but they're required to stabilize him to the best of their ability.
Cost -- $200k.
Billy Bob don't have a pot to piss in or a window to throw it out of. And he's gonna need lots more treatment. LOTS more.
LaQuan is on the corner, sittin' and spittin' and gets shot in a drive-by. He gets carried to the E-Room. Same scenario. LOTS of money spent to make him right and reasonable.
Who pays for this? You do. I do. We ALL do.
Either in the form of Cost-Shifting or in Welfare Dollars.
But if they had spent THEIR OWN MONEY to buy Health Insurance, like they should (but won't), then there wouldn't be any cost-shifting or Tax dollars spent on Medicaid, etc.
Of course, you have to understand that it might take 10,000 Billy Bobs and LaQuans participating in Health Insurance ot cover the one that is unlucky. Actuarial Tables and all that.
And let's not pick on the young.
Mature Adults get REALLY pissy when they're forced to buy Flood Insurance.
"I have lived here for forty years and never been flooded out!!"
To which a good reply is, "I have never been in a Plane Crash either. You only need one"
Then they get really pissy.
But people WILL NOT DO IT unless they're forced to. Or at least not enough people will do it.
It's why the Federal Government had to take over the Flood Insurance Program. Because only THEY have the power to FORCE you to buy it. Private Companies don't. It's ALL through FEMA.
Just human nature.
And I am one of the most Conservative people in here. But I know wherefore I speak on this topic
Sorry for the long post