But you're still using NAT and SNI for IPv4 hosts, so there's little incentive to add the complexity of IPv6.

Yes, but then you get native connectivity for “free.“ Turning your argument around, there is little reason to deal with the continued complexity of IPv4 (requiring NAT and SNI) just to talk to the remaining global NAT front-ends when it can all be skipped for direct communication over native IPv6.

BTW, NAT doesn’t scale forever. There are often several layers of NAT in carrier implementations and the port mapping issue alone can dictate the maximum number of clients-per-global-IPv4 address. One of the reasons NAT and IPv4 can still work is because much of the world has shifted to IPv6.