Do you have any examples of this happening under other UHC systems?

Russia. Healthcare in the far away regions or even somewhat remote villages is basically non existent. Pffff I lived in a relatively large city 4 hours away from Moscow, and there was a village about 30 minutes away from there. No ambulance would ever get there. There was no healthcare there. Russia is a textbook case of how universal healthcare can very easily and very quickly go very wrong.

Economies of scale and healthcare

Well a LOT of that administrative work is being mandated by the state - if the goal was to reduce it, wouldn't it make more sense to allow the industry to liberalize and get together under a single standard the same way every standard war ever has eventually ended?

The US is a controller of its own currency. It literally cannot go bankrupt because the money it owes is denominated in USD.

Wat. Do you propose devaluing the US dollar instead? I'm sure that this will not impact the US economy any more than a potential default would. Like, can't think of a single country, especially not one in the Western hemisphere a bit to the south of the US, where that happened.

If private insurance suddenly became medicare for all...

And if grandma had wheels, it would not be grandma, it would be a bus. It won't. Magic is not a thing. Implementation will take precious time, there will be administrative hurdles to get over, there will be competing standards, there will be problems. Saying there won't is disingenuous at best, and, if you ask me, an outright lie. That happened every other time government decided to meddle in healthcare and it will happen again.