> They don't lose any investment in IPv4 address space

What investment? IP addresses used to be free until we started running out, and I don't think anything of value would be lost for humanity as a whole if they became non-scarce again.

> they don't have to upgrade to all IPv6 supporting hardware

But they do, unless you're fine with maintaining an implicitly hierarchical network (or really two) forever.

> It's basically better NAT

How is it better? It also still requires NAT for every 4x host trying to reach a 4 only one, so it's exactly NAT.

> that eventually disappears

Driven by what mechanism?