It hasn't worked out how we thought, sure. But "Failed" is really strong for something that 1/3 of the global Internet can (and does) use every day.
There are more people on IPv6 now, than there are users in the entire US Internet market.
https://stats.labs.apnic.net/ipv6/CN (replace last 2 chars with any ISO3166 code, or XA for the world)
IPv6 has quirks which are a bummer. If we were able to fix the MTU problem by getting people to move to Jumbograms a lot of these go away, because they stem from fragmentation. So it's true, the design of IPv6 end only fragmentation distinct from V4 on the path fragmentation, that was a failure in design. But if you avoid fragmentation (and QUIC does for instance) then the MTU problem isn't a big deal in the end. More packets is faster clock pacing!
I wish people were a bit more pragmatic and a bit less dogmatic about things. Fix MTU. fix links. Fix what you can. Deploy IPv6 because it works. And, it reduces pressure on your CGN.
i tend to regrad ipv6 as the "smartphones + india internet"
Above the IP layer, TCP/UDP rules should be underlying AFI agnostic. But I can believe the firewall DSL don't make that easier.