It's not a full rewrite, just an extension The majority of packets on the internet pass through fairly hard-coded switching chips that probably couldn't be upd

It's not a full rewrite, just an extension The majority of packets on the internet pass through fairly hard-coded switching chips that probably couldn't be updated to handle this. They can handle IPv6, though. If it were made truly backwards compatible It can't be made backward compatible. Every switch on the internet needs to understand your new extended address scheme otherwise they'll send your packets to a dead-zone somewhere in Uzbekistan. Backward compatibility was never an option. As soon as you add new bits to the address, old routers won't send them to the right place.