I'm not confused about the NAT / firewall distinction, but it might be nice if my ISP didn't have a constant, precise idea of exactly how many connected devices I owned. Can that be _inferred_ with IPv4? Yes, but it's fuzzier.
Aren't your home addresses assigned by your local router?
The ISP still doesn't know how many devices are connected, because a lot of those devices are using randomized and rotating IPs for their outbound connections.
Is this solved by the device having between 1 and X randomly generated IPv6 addresses?
Some of my devices have 1, some 2, and some even more. Takes some precision out, at least.