v6 is about as backwards compatible with v4 as it's possible to be. If you have a way to make it more backwards compatible then I'd love to hear it, but when I ask this all I ever get are things that don't work, or things that v6 already does.
But for another, the v4 space is available as a subset of the v6 space:
$ ping 64:ff9b::8.8.8.8
PING 64:ff9b::8.8.8.8(64:ff9b::808:808) 56 data bytes
64 bytes from 64:ff9b::808:808: icmp_seq=1 ttl=113 time=9.82 ms
127.0.0.1 needed to be a valid IPv6 address, along with all the others. Pick a particular prefix, say 0...* and any address with that would be extended to 128 bits. That would have been backwards compatible.
There is no possible way to design an address protocol with bigger addresses than v4 that a) makes v4 forward compatible with it, and b) can actually work. Feel free to suggest one.
> .0.0.1 needed to be a valid IPv6 address, along with all the others. Pick a particular prefix, say 0...* and any address with that would be extended to 128 bits
That prefix is ::ffff:0:0/96. 127.0.0.1 is ::ffff:127.0.0.1 (::ffff:7f00:1). 30 years and you still haven't realized v6 has this?