> If you can't update a system, you have no business exposing it to a network. Full stop.

What about isolated networks, a single Ethernet switch for example, used for industrial automation? We can't keep replacing equipment every 5 years because they keep changing crypto protocols.

Many industrial networking protocols have no security at all. No encryption. Not even a password. They achieve their security by being disconnected from the outside world, constrained to a single room or building. If an attacker can get physical access to the wires, then we have a lot more to worry about than network security.