> It's still just one firewall.

One appliance or service, but double the rules. The rules are all of the maintenance cost

> You can listen on a single socket too (sockets listening on :: will accept v4 connections by default on Linux).

Old apps need migrating. 99% of apps that listen 127.0.0.1:PORT and need a rebuild & re-test. Any app compiled with AF_INET need a rebuild.

I encountered this working on adding ipv6 support for oauth callbacks (127.0.0.1:3000) to rclone and it was a huge pain. still never got this working reliably enough for the maintainer to merge.

You're thinking about your desktop where you are recompiling constantly. I'm talking about embedded & unsupported IOT devices that are out there. Even with sources the effort to rebuild reinstall is heavy.