NO!
> or rather, "assume every system on earth can/will be breached"
NOOOO! NO NO NO!! It's ALREADY BREACHED! You are using ADVERSARIAL CHANNELS! If we're talking about objectives and proofs for the purposes of operational security, the only way of going about this is to act as though every system you are using has chips in it the NSA put there. Please understand.
https://learn.microsoft.com/en-us/security/zero-trust/adopt/...
oh, sorry for misunderstanding then.
> NOOOO! NO NO NO!! It's ALREADY BREACHED!
i see and hear your point. it just makes me sad to think about security in this way, even though i think it would be good for someone that operates in the cyber security sphere. i think a more healthy way of approaching this for the general person, is what the person above said: "[...] what you're practically shooting for is to make the cost higher than the value an attacker would get from breaching you.", or for you not to lose more money/resources/value/whatever by securing your system, than you would do by getting breached or attacked. but that is just my opinion, and i am not an expert, so!
:o)