So:
* https://en.wikipedia.org/wiki/Teredo_tunneling
* https://en.wikipedia.org/wiki/6in4
* https://en.wikipedia.org/wiki/ISATAP
You can sign up for an IPv6 connection, without any IPv6 support from your ISP, via Hurricane Electric's tunnel service:
* https://wiki.archlinux.org/title/IPv6_tunnel_broker_setup
* https://docs.netgate.com/pfsense/en/latest/recipes/ipv6-tunn...
It's been around for at least a decade:
* https://www.internetsociety.org/blog/2014/01/weekend-project...
Any 'expanded IPv4 address' solution will (have) face the same problems that IPv6 has faced, and the solutions will end up being the same: having to slowly upgrade the network stack (hosts, routers, firewalls, ASICs), changes in networking system calls and associated structs (because the number of bits), gateways and transitions mechanisms, expansion of related protocols (DNS A records are only 32-bits, so a new record type needs to be supports (e.g., AAAA for IPv6, AAAAAA for your new proposal)).
If you want to argue that we should have (say) kept ARP instead of moving NDP, sure.
But anything to do with expanded addressing would be the exact situation: code upgrades and transition mechanisms.