The fact that it takes so long to transition to IPv6 and like many other obstruction to technological/social progress are economic and business gain for a certain small group of people and organisations.
The spread and reach of TCP/IP will accelerate at a pace faster than that of the transition to IPv6 indefinitely, therefore we are saddled with IPv4 until the end of time.
If I were to list impediments to the technical development of the Internet, local telco monopolies would be much higher in the list, but it's another kettle of fish.
Unless you were using Windows, where IIRC the IPv4 and IPv6 stacks were separate, and a single socket couldn't be used to listen to both at the same time. (This might already have been fixed in more recent Windows releases.)
Instead of that we got fragile, backward incompatible, unnecessarily complex, jack of all trades protocol. And people are surprised that even after decades since the release of IPv6, it is being avoided like a plague.
I mean, hosts need to be 'dual stack' to support IPv4 and IPv6 together - whereas TheLoafOfBread's proposal would allow support of both with one backward-compatible stack.
I would challenge you to find someone who knows the difference between Ethernet broadcast and multicast without looking up the answer. They are very similar mechanisms.
NAT64 is involved on the level of TCP & UDP, not lower level protocols.