> So it does provide practical protection to use NAT in that you've reduced the set of valid attackers from "anyone on the Internet" to "people with the ability to affect the upstream router in my ISP"
Except it really doesn't because you haven't?
There are two practically relevant circumstances where the statement "NAT provides security" is made:
1. With regards to real-world home routers (which also come with a stateful firewall), in which case the statement is simply false: Adding or removing NAT makes no difference whatsoever for security. This is also the scenario that then is used to argue against IPv6, because IPv6 supposedly lacks this security, when it just doesn't, because the IPv6 router also comes with a stateful firewall, and adding NAT (or disabling IPv6 because of a "lack of NAT") achieves absolutely nothing security-wise, because there is no problem in the first place.
2. By people who falsely believe that NAT does in fact prevent all inbound connections and for that reason fail to configure the necessary firewall rules on more business-oriented routers, so the belief that "NAT provides security" keeps them from actually securing their network, and that in particular in scenarios where it is much more relevant than for your typical home user.
So, it is technically true that NAT could have security-enhancing effects under artificial circumstances. But under all real circumstances, it either just doesn't, or the belief that it has is what causes the configuration to be less secure than it easily could be, if only people didn't have the completely not-understood belief that "NAT provides security".
Except it really doesn't because you haven't?
There are two practically relevant circumstances where the statement "NAT provides security" is made:
1. With regards to real-world home routers (which also come with a stateful firewall), in which case the statement is simply false: Adding or removing NAT makes no difference whatsoever for security. This is also the scenario that then is used to argue against IPv6, because IPv6 supposedly lacks this security, when it just doesn't, because the IPv6 router also comes with a stateful firewall, and adding NAT (or disabling IPv6 because of a "lack of NAT") achieves absolutely nothing security-wise, because there is no problem in the first place.
2. By people who falsely believe that NAT does in fact prevent all inbound connections and for that reason fail to configure the necessary firewall rules on more business-oriented routers, so the belief that "NAT provides security" keeps them from actually securing their network, and that in particular in scenarios where it is much more relevant than for your typical home user.
So, it is technically true that NAT could have security-enhancing effects under artificial circumstances. But under all real circumstances, it either just doesn't, or the belief that it has is what causes the configuration to be less secure than it easily could be, if only people didn't have the completely not-understood belief that "NAT provides security".