Back before NAT was a thing, technically-inclined people like us would seek out firewalls on our computers. Nowadays most people have them in the form of with Windows Firewall and IPTables, but many people don't really understand how to use them because they're not as necessary as they were. I would imagine most people's interactions with Windows Firewall have just been to designate which networks are private and which are public, and only then because Windows automatically asks you when you connect to a network.

With IPv6, that's going to change again, and firewalls are going to become much more important once more - but It would explain why people can run without firewalls, right now. After all, currently the only things they're protecting against are from your local network, since your router is probably doing most of the external blocking already with NAT.

Of course, this doesn't help IPv6 adoption at all...

Not sure, depends on your threat model and how you organized your LAN.

I put my “trusted” devices in a special vlan and untrusted devices (IoT) in another one. Only trusted devices can access IoT devices but not vice versa.

Works pretty well.

But yeah that does require a more advanced router / switch as well.