By that logic, you can disable IPv4 and still access the site via IPv6.

IPv4 is useless.

Also, running dual-stacked servers I can confirm, all scans come via IPv4, so not only it is useless, it’s actually less secure to use it.

By that measure, port 22 is useless because you can change it....

How about all of the people who wouldn't be able to access the Internet because their country doesn't have enough v4 address. Or who's internet connection is better/faster because they're no longer behind a CG-NAT. (Think mobile carriers, developing countries, China, India, etc)