Simplifying address space assignment is a huge deal. IPv4+ allows the leaves of the network to adopt IPv4+ when it makes sense for them. They don't lose any investment in IPv4 address space, they don't have to upgrade to all IPv6 supporting hardware, there's no parallel configuration. You just support IPv4 on the terminals that want or need it, and on the network hardware when you upgrade. It's basically better NAT that eventually disappears and just becomes "routing".
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?
The removal of arp and removal of broadcast, the enforcement of multicast
The almost-required removal of NAT and the quasi-relgious dislike from many network people. Instead of simply src-natting your traffic behind ISP1 or ISP2, you are supposed to have multiple public IPs and somehow make your end devices choose the best routing rather than your router.
All of these were choices made in addition to simply expanding the address scope.
Only use the real one then (unless you happen to be implementing ND or something)!
> The removal of arp and removal of broadcast, the enforcement of multicast
ARP was effectively only replaced by ND, no? Maybe there are many disadvantages I'm not familiar with, but is there a fundamental problem with it?
> The almost-required removal of NAT
Don't like that part? Don't use it, and do use NAT66. It works great, I use it sometimes!
(i.e. anything other than the decision to make a breaking change to address formats and accordingly require adapters)
And if the fellow you're replying to doesn't know that broadcast is another name for "all-hosts multicast", then he needs to read a bit more.
[0] Several purity-minded fools wanted to pretend that IPv6 NAT wasn't going to exist. That doesn't mean that IPv6 doesn't support NAT... NAT is and has always been a function of the packet mangling done by a router that sits between you and your conversation partner.