> Private IP space is incredibly useful. I build it and set it up -- my ISP does not have control.

You can have that with IPv6, too. You can even get your own ULA prefix that (hopefully [1]) only you will ever use: https://ula.ungleich.ch/

[1]: Technically, it doesn’t prevent anybody else from using the same space as you. (And you can’t advertise it, of course.)