Backward compatibility was never really the problem; the problem is that forward compatibility with ANY successor protocol (without modifying IPv4) is a fundamental impossibility.

But at least a reasonable facsimile eventually came out with NAT64.

(You can also do NAT46, but it requires one IPv4 address for every IPv6 destination you want to be reachable from the IPv4 Internet, so it doesn't scale very well.)

Corporate IT networks have less IPv6 and residential/mobile networks have more IPv6, so on weekdays when people are using Internet at work = more IPv4, weekends when people are using Internet at home = more IPv6. Christmas also has a big bump for the same reason.