NAT64 and DNS64 would allow for deployment similar to what I have described (starting from the opposite end, with the new protocol on the client side, implemented first by ISPs that have lots of clients), and I hope that is actually what ends up happening. The problem with doing this is that all the devices on the client end of things need to handle IPv6, and due to the need for all the configuration/administration to be duplicated for no apparent benefit, there are a lot of devices out there that don't currently support (or at least don't enable) IPv6
for embedded devices, having to run dual stack is considerably more expensive and complicated than just running IPv4, a tweak like I suggested would have been pretty easy to add into a IPv4 stack, especially in comparison.
however look at the dates involved, NAT64 is a pretty recent development, within the last couple of years.
IPv6 and it's 'migration plan' is 20 years old, for almost all that time, any suggestion to implement anything like NAT64 would get shut down by the IPv6 people as being against the migration plan.