What you're describing is essentially 6to4, minus the anycast gateways. The problem with using 6to4 as the IPv4 upgrade path is that it solves the easy problem

What you're describing is essentially 6to4, minus the anycast gateways. The problem with using 6to4 as the IPv4 upgrade path is that it solves the easy problem (pushing big addresses through the core of the network), but not the hard problem (updating all the edge devices, and every piece of software that stores or manipulates an IP address.)