>much of what USPS does is unprofitable

The same could be said about many organizations within companies if you don't give them a proper budget. Once you start actually caring about being profitable it turns out that you can find how to do things in a way that is less wasteful. Cost acts as an incentive to reduce waste and if you remove it then there is no force to combat waste or unsustainable practices.

>It’s not that private entities won’t deliver postal service; it’s that they quite literally can’t.

If you paid someone $10,000 to deliver a letter to somewhere in the country I'm sure they could find a way to bring it there. It is not impossible.

Yeah, of course they can/could, it's just that it does not make sense from an economic point of view. Which directly circles back to the beginning "USPS is not profitable".