The advantage is that you can be behind a dozen layers of NAT on the v4 side and it doesn't matter so much because 99% of your traffic is going over v6 anyway a

The advantage is that you can be behind a dozen layers of NAT on the v4 side and it doesn't matter so much because 99% of your traffic is going over v6 anyway and anything that needs an inbound connection has the v6 available instead of being impossible. It's okay for v4 to suck if you can avoid it most of the time, but not so much when it's your only option. Maintaining v6 is a lot less work than site-to-site VPNs, especially when "specific parties" is "random internet users" and when RFC1918 clashes are a thing. v6 is worthwhile even if there is still some v4 going on. And even if it wasn't, how would you propose to deploy v6 and turn off v4 in one internet-wide atomic operation? There's no longer any entity that can enforce a flag day on the internet, so I can't see any way of doing that. The result is that we'd need to do a gradual changeover, which happens to be exactly what we are doing.