That would still constitute an entirely new system, just one that looks very like the previous one. So much code and other infrastructure (routing tables in hardware etc.) relies on IPv4 addresses being 32-bits not 64 and that would all need to change, complete with extra new stuff being added for that translation layer while both are in operation (which would end up being near permanent, people will find ways to keep the old stuff running as long as they can). I expect that this new scheme would just become a new version of CGNAT, “Global NAT” perhaps, with the same problems that adds.

IIRC something like that was considered when IPv6 was first being designed, be deemed insufficient for the long-term future. 64-bits is thought to be enough to stave off address starvation though, which is why IPv6 address ranges are usually assigned as /64s (my homer link has a /48 range should I wish to use it all!), so I can see your reasoning on that level.