If you're operating a public site and you switch to IPv6-only, you'll no longer be reachable by the majority of the current Internet. That's a pretty big sacrif

If you're operating a public site and you switch to IPv6-only, you'll no longer be reachable by the majority of the current Internet. That's a pretty big sacrifice for most people. If you're operating a public site and you don't switch to IPv6-only, for example by doing dual-stack, you'll still need IPv4 addresses. The challenge is that "just switch to IPv6" isn't something that you can do unilaterally if you need to talk to the rest of the Internet.