That is the sort of selfishness the GOP has been indoctrinating people with for 45 years: "why should I pay someone else's medical bills?", "why should I pay to educate someone else's spawn?"

It's not selfishness. It's perfectly reasonable to be upset that one person would have their health care paid for them for their entire life, while you have to pay for your own health care and pay the taxes to pay for theirs, for no reason other than an arbitrary date. People are often happy to pay for other people's needs: for children or for the poor or for the disabled, who are unable to provide for themselves. Or for everyone. But when it's for some and not others based on an arbitrary reason, it seems (and arguably is) deeply unfair.

The problem is not insoluble, no matter how much insurance industry lobbyists insist it is.

Attributing it to insurance company lobbyists is a deflection. It's insoluble (or nearly so) because there are lots of different groups who would be adversely affected by the transition to a single payer system, depending on how it's structured. You'd need either a massive payroll tax increase (which hurts most workers), a massive VAT (which hurts retirees among other voters), or a massive cut in provider compensation (which...well, obviously). Or all three. Insurance companies hate it too, but there's enough of those other folks. Which is why these efforts have foundered even in blue states, and even in blue states where insurance companies aren't much of a presence.