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.)
I think most of us know that their design failure here was a lack of backwards compatibility. But at least it's getting adopted.