Perhaps, but it turns out not to be possible to extend v4 without producing something that looks like v6. You can only fit 32 bits into the dest address field i

Perhaps, but it turns out not to be possible to extend v4 without producing something that looks like v6. You can only fit 32 bits into the dest address field in the v4 header, and any attempt at working around that problem ends up looking like v6 plus one of the existing transition methods (NAT64/6to4/Teredo/etc). If the biggest problem in v6 is one that fundamentally can't be fixed, then I think we did pretty good.