It's been too long
If IPv6 hasn't taken over by now, it'll never be widespread. It's actually obsolete -- it was developed 20 years ago, and the Internet has completely changed since then. IPv6 was actually designed as an alternative to IPv4, not an extension. As a result, there was no defined upgrade path, so nobody upgraded to it and nobody completely dropped IPv4 support.
Also, it's a lot easier for an end user to securely configure ONE device instead of the dozen or so devices that are in his house. As a result, he (and everybody else) is hiding behind NAT and the firewall in his router. Yet IPv6 essentially eliminates NAT, which is the #1 Internet security device in use today. Twenty years ago, security wasn't a problem. Today, if an end user connects an unprotected device directly to the Internet, it will be hacked by the time he downloads, installs, and configures his firewall.
My router and ISP support IPv6, but all the devices connected to that router are IPv4 with non-routable addresses. This is how most people have it set up, even though they probably don't know it. Until their router crashes from all the juggling going on, at which point tech. support will tell them to configure the router to be IPv4 all the way (like I did).
I hope the next iteration is just IPv4 with more bytes in the IP address.