Second system effect. It's way more complicated

Replicating even a simple ipv4 lan with mostly stable local addresses and some isp uplink is pretty complex, even assuming all the devices involved support ipv6, which they wont

It's much easier to run a dual stack with v4 for ease of use on the lan and v6 as an additional address that can change whenever and is only used to reach the wan

But the main reason you'd care would be if you can't get a public ipv4 address to NAT to. And if you can't get that, some of the internet breaks anyway.