double the firewall, double the listening sockets to manage, double the testing (e.g. my router was working ipv4 and broken ipv6 with the same daemon), double the app-level ACLs

You can argue "it's only one line" but that one line is a new socket and new test variant needing testing. something that worked perfectly well for 5-10 years now needing a re-test.

I'm not arguing against ipv6 . I'm arguing for honest assessments of the effort needing to migrate a network , especially residential networks, to IPv6 -- as the only way to make it happen. Shaming people with "it's so easy and simple" is just dishonest and doesn't help the cause.