Why? It would be to the benefit of relatively few and be an enormous technical problem for decades for everyone. If we did this, the IPv4 addresses would run out again after a few months at most, leaving us all in the same position we are in today, but also with a huge technical problem of fixing all the old devices which had these now-repurposed networks hardcoded.

If you are fine with doing the work of obsoleting old equipment, then just start using IPv6.