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That side effect of NAT is easily replaced with a simple firewall. Even today, many home routers have this capability already by virtue of running linux. Enabling it would be a pretty simple step for manufacturers.

You need state tracking to build a masquerading NAT (or it won't know which machine to route reply packets to), and if you have state tracking, you can also build a stateful firewall, which will achieve the same thing.

Stateful firewalls still work, have always worked, and work the same in IPv6 as they do in IPv4. Having a public, globally-routable, unique address on your internal machine, whether that's an IPv4 address or an IPv6 one, doesn't mean that anyone can connect to it. It still has to go through your router. That router can be running a stateful firewall.

NAT is awful.

NAT is not a firewall, it's a hack to keep ipv4 working today.