It seems like this is the same world as we're currently in. IPv4 hosts can contact each other and can't contact IPv6 hosts. Hosts which are dualstack can be contacted over either stack.
The problem is the incremental effort (not just software wise, but administrative as well) needed to achieve this outcome. This comment explained it much better than me:
https://news.ycombinator.com/item?id=37555424
