But I'd claim it failed completely. Many very public services (eg. github.com, sentry.io, office365.com, archive.org) outright don't support it. And the fact that infrastructure providers (like mobile network carriers) don't support it either, proves that it is far from "alive". In fact, I'd say it is deader than MMS, Fax and landline. For these I can still get service. For IPv6 (on the phone) I can't.
Email didn't replace letters on paper. But I wouldn't say email failed completely.
Given the number of scaredy-ass ISPs out there who refuse to offer IPv6 service, I disagree with this. But I do strongly agree with the rest of your comment.
I also very strongly assert that claims of the form "IPv6 is not the only version of IP used on the Internet, therefor it is a complete failure." maybe with a side of "And this 'failure' means that IPv6 is a dead protocol." are just plain stupid. v4 and v6 were very deliberately designed to coexist, and if v4 continues to run on the Internet until the day the Internet is shut down, that's totally fine.
In my opinion, the most plausible future is one I mention in my second paragraph here. [0]