That still doesn't work though, because an IPv4 host still can't address an IPv6 host, and an IPv4 router still wouldn't be able to route to an IPv6 network (wi

That still doesn't work though, because an IPv4 host still can't address an IPv6 host, and an IPv4 router still wouldn't be able to route to an IPv6 network (without hacks far uglier than NAT). And they couldn't just upgrade every router in every backhaul network to the new protocol at once (assuming what you describe could actually work) - that took a decade, because lots of it used custom silicon for routing and switching.