They actually did that (see sibling replies), and it actually delayed adoption of ipv6 by a few more years, because of things like: 1. some OSes can listen o

They actually did that (see sibling replies), and it actually delayed adoption of ipv6 by a few more years, because of things like: 1. some OSes can listen on ipv6-mapped-ipv4 automatically when some application listens on ipv4 2. app is updated to listed on both ipv4 and ipv6 3. at various companies, the updated app fails to start, because it listened on ipv4, which automatically listened on ipv6, and then it tried to listen on ipv6 and failed because the port was already in use 4. The problem is weird and confusing, and the easiest fix is to just disable ipv6 everywhere: in the app, in the OS, everywhere. Then the app works again. 5. The config changes remain untouched for many years (subtle config issue, don't touch!) I think ipv6 would be more widespread if various auto-adaptation mechanisms were never introduced: 6to4, teredo, etc. I personally experienced this in my teens, one day my browser took 30 seconds to start loading any web page, and just disabling ipv6 everywhere fixed it, and for the next couple years browsers disabled ipv6 by default (whereas they previously were agnostic), and macOS also did a similar thing where it reduced ipv6 enablement by default for a few years, also domains like google.com and facebook.com removed AAAA records they previously had for a few years. They just can't tolerate 1% to 3% of users breaking due to ipv6 mis-config. Adoption would have been much smoother and faster overall, without these significant contributors to automatic-possibly-broken ipv6. (Yes IPv6 works fine now, with happy-eyeballs, and DHCPv6, and pretty much only real-native ipv6 used anywhere)