There are multiple v6 prefixes already allocated for containing the v4 space. It would help if all ISPs ran NAT64 routers on the standard NAT64 prefix, but e.g. how would that function with software that only works with v4 addresses?
OS would translate the ipv4 address to ipv6 before it leaves the machine. Very rare that software writes its own packet to the network card.
On an ipv6 only device if I type "ping 1.1.1.1", I would expect that to be translated by the OS (not the program) to 64:ff9b::1.1.1.1, rather than relying on DNS64 to do it.
Then at some point upstream it would go via a nat64 gateway to reach that legacy IP.
Then you're back at having "two internets" again. (Well, I don't agree that it's two Internets, but you'd be stuck with the same situation we have already because people would continue to use v4 in order for that software to continue to work.)
There are multiple v6 prefixes already allocated for containing the v4 space. It would help if all ISPs ran NAT64 routers on the standard NAT64 prefix, but e.g. how would that function with software that only works with v4 addresses?