Ok so try calling my phone at sip://ninkendo.example. Oh, I’m actually not at my house. I’m traveling and my device is moving from one cell tower to the next, getting a new IP address every few minutes. Then I get to my friends house and his router is gonna block unestablished connections so you better call me soon.
No, there’s a reason software like FaceTime, and calling in general, are mediated by centralized services and implemented as client-server. Because relying on your users to have stable IP’s, and to always be behind a properly configured firewall, would be an utterly insane way to design something.
Hence “ipv6 is a relic of a bygone era”, because in the era we live in now, IP addresses change and users move between networks constantly, and NAT is very much the least of our problems.