IPv6 is actually working right now for many so I wouldn't call it a failure. Use of IPv6 has been increasing over time and who really cares how fast that is happening? Some ISPs, nodes or networks may never use it - but that's OK; really as long as major cell phone networks are using it, it's not going away.

Windows XP is another example of something that lingered far longer than it should have. It's not a failure just because everyone didn't immediately start using it.