Maybe not in the strict sense, but it kind of has.

In the enterprises I've worked in the past decade with IPv6 running, at least 75% of the Internet traffic is IPv6. In my discussions with other engineers managing large networks, they seem to be seeing more or less that same figure.

The problem is that virtually nobody knows IPv6. I regularly bring up IPv6 in engineers' circles and I'm often the only one who knows much about it. And so, I have doubts about it's long-term future, except for edge cases. I figure some clever scheme utilizing IPv4 and probably NAT will come around at some point.