[lmm]

Mandating that every router has to do stateful connection tracking would have been an enormous, wasteful burden. NAT64 is there for those who need it; 464XLAT setups with IPv6-only clients are quietly the reality on networks that don't have too much legacy infrastructure (mostly mobile).

[octoberfranklin]

And having two completely separate internets (IPv6 and IPv4) forever isn't a wasteful burden?

Not requiring backwards compatibility in IPv6 guaranteed that IPv4 would be around forever. IPv4 is never, ever going away because of this.

The only people who couldn't see this coming were from Bell System backgrounds where you could use centralized schemes like "Ma Bell says tomorrow is the Flag Day, flip the switch". In a decentralized system people don't stop using the old system until you give them a new system that is backwards compatible. Then you drop the backwards compatibility in a second, separate upgrade much later, on a timetable dictated by adoption, not flag days.

[lmm]

> And having two completely separate internets (IPv6 and IPv4) forever isn't a wasteful burden?

Your proposal would force maintaining IPv4 for longer and in more networks: every IPv6 router would have to have IPv4 connectivity and probably a routeable IPv4 address, so it wouldn't even solve the address exhaustion problem for long (perhaps not even at all).

> Not requiring backwards compatibility in IPv6 guaranteed that IPv4 would be around forever. IPv4 is never, ever going away because of this.

IPv4 has already been eliminated from newer edge networks, and for those networks the vast majority of upstream traffic is IPv6. No doubt those networks will have to maintain 464XLAT for a long time as the long tail of upstream sites that are only v4-accessible, but they'll be able to have a smaller and smaller pool of 464XLAT servers and outsource the v4 connectivity support further and further upstream (just as with Usenet), until eventually v4 connectivity becomes a paid add-on and then goes away entirely. Home routers for use with PCs will probably have to offer 4over6 for a long time, because it's hard for an ISP to be confident all their users are up to date, but that doesn't actually reduce the benefits that much (all your internal network management can still be v6, only the little home user LANs are v4), and organisations that manage all their endpoint devices don't even need that much.