The technical solutions will not work, as there are many, many possibilities to make a mistake. Misleading router UI, copying the configuration from somewhere e

The technical solutions will not work, as there are many, many possibilities to make a mistake. Misleading router UI, copying the configuration from somewhere else and forgetting to update it, moving a computer to another network, just forgetting to enable a firewall (in many cases it is impossible to set up a IPv4 network without a NAT precisely because IPv4 addresses are rare — but it is perfectly possible to do it with IPv6, and it will be insecure by default). Secure systems are robust against user mistakes (and even middlemen mistakes). A NAT is one such system. Alternatives do not work like that.