Retiring IPv4 was never a strong goal of IPv6. The "inter-" in internet was always about "interoperability" between networks and side-by-side networks that inte

Retiring IPv4 was never a strong goal of IPv6. The "inter-" in internet was always about "interoperability" between networks and side-by-side networks that interoperate was always the plan. Dual stack IPv4/IPv6 was in part chosen because it doesn't sweep the interoperability problems under the rug like some of these old "IPv4 extended" proposals did. You are never going to upgrade all devices at once and you were always going to have two networks, it was just a little bit easier with some of the other proposals to pretend it was one network if you squinted really hard and ignored all the interoperability problems that would crop up in practice because devices didn't update to understand the extensions.