Look at the deployment of TLS as a success story. Everyone kept on supporting both old and new, watched the percentages, and then dropped the old when the new had enough penetration.
The whole thing is also a bit of a shell game. Nobody wants to invest in it until they feel like they're "behind" if they don't. So you have a big player or two in order to make it feel like that's the way the wind is blowing.
Eventually some protocol is the bottom of what a group agrees to speak rather than individuals and that protocol has a completely different set of deployment issues than abstraction layers that can be built between end stations at higher levels.
The biggest problem with IPv6 is that it a different protocol that requires special support on all levels starting from applications and down to managed switches.
Has WiFi been upgraded? I believe new devices still have support back to 802.11b.
IPv6 is a great example of how not to.
There are a handful of browsers that pushed the adoption. There's nobody doing the same in the IPv6 space.
There are areas that the browsers purposefully ignored (DNSSEC, DNS-SD) and thus their adoption is low.
And brings out the problem with IPv6- there is no sector pushing IPv6, because no sector sees the benefits.
Question is why does no sector see it? My theory is that it adds a lot complexity and planning for little gain. So no sector is going to push for it.
For ISPs, it means investments with no benefits. They do it only once their CGNAT is too overloaded or on other sign of running out of IPv4. When they do it, in the simplest and cheapest way possible, which the informed customers see as worse service, so naturally they want to avoid it.
IPv6 would help network application developers, so they could do more or do what they do now with less, but they have no impact on investments ISPs would have to do.
Then there are parties, that have working IPv4 infrastructure, feel no pressure caused by lack of IPv4, and migration would mean just expenses with zero benefits. Exactly like companies that ran login forms over http, up until browsers started to warn users. That was the incentive, that caused them to switch to https.
ASCII -> UTF8
VHS -> DVD
not as successful:
DVD -> BluRay : timing was rather close to the rise of streaming services like Netflix, Hulu, etc.
1. DVD video quality is good enough for most.
2. The Combination of DRM, unskippable portions, and similar is enough of a barrier to discourage the upgrade. Personally, I will only watch a movie on disk when I don’t have time to rip it and remove that crap.
I'd be more interested in how the HD DVD as a very visible competitor blocked BluRay adoption because of consumers wanting to wait for a winner in that war.
This assumes markets with little to no competition. But most ISPs are in competitive markets with no trend setting big players and they can't waste money investing into IPv6 or anything else that has no demand if they want to keep the business alive and healthy.
From the client perspective there is the same level of compatibility between IPv4 and IPv6: client can connect to both types of addresses.