FWIW, you could drop the words "computer" from your first paragraph and it would still be true. People doing security outside of "cyber" know this.

If you have ${anything}, assume it is semi-public, meaning if someone was interested enough in accessing it, they could do it. "Fort Knox" isn't the right analogy for physical security; the primary question isn't whether something can be breach, but how much would it cost the attacker to do it.

A system where expected costs to attackers >> expected profit they could reasonably make from succeeding, is considered secure in typical case (exception: special cases where attackers may be driven by non-material reasons - think terrorism, politics).

Computer security is largely still stuck in "Fort Knox" thinking.

> For example, do not hook your goddamn water or traffic or electricity infrastructure up to the goddamn Internet, and then, do fire the guy who suggested it.

That ship has sailed. "Modern problems require modern solutions", that infrastructure will be in some way accessible over network is a necessity at this point, the question should be, how to keep it difficult for normal attackers to mess with it, without preventing the system from fulfilling its intended function.

> The correct analogy for computer security is not locks and keys and doors and gates. It is a house in a floodplain. Your house will not survive the flood of it hits you. Do not store anything critical or irreplaceable in that house.

The correct analogy for computer security is a house. Scale security proportionally to actual importance of what's inside, and accept that nonzero amount of houses will be broken into; that's just insurance writeoff. Leave the 20-meter walls with towers and armed guards and helicopter gunships on fast-dial for the critical junctions, while keeping in mind that this is not perfect either - it won't stop an open nation state attack, at best maybe slow it down.

For critical infrastructure, I'd honestly focus on redundancy, resiliency, limiting blast radius and procedures to recover quickly, over trying to turn every physical or virtual substation into unpenetrable fortress.

(Another thing physical security gets right, that cyber side seems to ignore: security does not and cannot exist in isolation; criminal justice system and law enforcement are part of it, and at extreme end, threat of military intervention against a state that aids and abets the perpetrators. The possibility of sending "men with guns" after perpetrators is core part of securing a system or space, it's literally what they are for and why we fund it with taxes.)