> I wouldn't say "failure". There are many, many IPv6 client devices out there, mostly on mobile networks.

Honestly it's a huge success due to this fact alone.

IPv6 is failure only if you measure success by replacing IPv4 or if you called "time" on it before the big mobile providers rolled it out. The fact that all mobile phones support it and many mobile networks exclusively deploy it tells you what you really need to know.

IPv6 is a backbone of the modern Internet for clients, even if your servers don't have to care about it due to nat64.