Making ipv4 a second class citizen working like an low performance compatibility layer for legacy seems like a great migration plan to me. Especially if performance would go down progressively.
The backbone network is unlikely separated by hardware for IPv4 and IPv6. If one drains resources, the other gets effected.
They could start to charge differently for IPv4 traffic and IPv6. If peering Terms of IPv6 interconnection agreements was free/radically lower than ipv4 (by say, increasing ipv4 charge rate), ISPs would see a direct encouragement to move to ipv6. Its not a unique concept, as similar suggestion has been made in 2011 (http://www.canscouncil.net/presentations/CANS2011/china_ipv6...)
