But now we have most of the content providers, CDNs, social networks, etc., already supporting IPv6. This is probably more than 50% of the traffic.
If we deprecate now Happy Eyeballs, it will take probably 6 months/average, I guess even one year, to get it dropped from browsers, OSs, etc., which means that meanwhile IPv6 support keeps growing.
When people start experiencing problems, it is either a bad IPv6 deployment at the content provider or the ISP, and they need to FIX IT, not ignore it because nobody notice it.
I doubt that any software vendor will make that change. You cannot go from something that sort of deals with failure, to something that breaks. Of course, users will notice right after an upgrade of the software, so they know who to blame. Early on with RIPE Atlas we had an issue with IPv6 PMTU blackholes. And I had the same attitude. Don't do anything on our end, just tell probe hosts to fix their connections. That didn't work out at all, so now we just clamp the mss on our end. Next step, if you would do this, users will quicky find out that disabling IPv6 will solve the issue. There is already too much 'when in doubt, disable IPv6, it is useless anyway' going on. Finally, it is nice to think ahead, but maybe thinking about getting rid of IPv4 is a bit premature? A couple of months ago, my home network had no IPv4. Time to see what my normal setup would look like on IPv6-only. Facebook worked. So at least I could tell my friends that I was still alive :-) My home network is a bit more complex than that of the average consumer, so as I already expected, Google was completely dead in the water. And of course, most the other websites that I frequently visit were unreachable. So maybe we should wait until going IPv6-only is actually realistic. And until that time think about minizing IPv4 traffic and not so much about breaking stuff when IPv6 doesn't work as expected. Any ISP can use flow data to figure out which content providers generate a lot of IPv4 traffic and talk to them about improving their IPv6 support. Any sensible happy-eyeballs implementation will then prefer IPv6 and IPv4 traffic will drop.