> I guess a lot of existing universal-coverage systems, such as the UK's NHS, are also single-manager (someone will correct me if I'm mistaken), but the very existence of Medicare Advantage shows an alternative possibility. I think that, far from being an argument against single-payer, it shows how single-payer can be made to work even better, through premium subsidies (and, I would guess, reinsurance for catastrophic losses) to private insurance companies, for those who are willing and able to pay additional premiums for better care.

I don't think you understand how Medicare Advantage works. There's really nothing about it that suggests a single-payer system would work. The premiums are not subsidized. (Reimbursements are, but that's a separate matter.)

Medicare is the fullest extent to which people can legally opt out of Medicare, short of claiming religious exemptions (rare), working for a foreign government while in the US (rarer and legally questionable for citizens), or surrendering US citizenship altogether (drastic). Medicare Advantage outperforms Original Medicare on the top performance indicators (medical outcomes, cost, patient satisfaction), though underperforms the same exact private insurers on those same metrics, despite the latttermost also being saddled with the burden of subsidizing Original Medicare reimbursements.

If you still want to advocate for single-payer healthcare on philosophical grounds, okay. But from an empirical standpoint, Medicare is not the place to look for data to support that stance.