It really depends on your threat model. It is not always unreasonable.

Target trusted their HVAC management firm so much that they had full unsegmented access to the LAN in each store. The credit card swipe terminals in the same LAN were totally compromised and millions of users had their credit card credentials stolen.

Defense contractors and places that store / manage large amounts of money are totally within their mandates to trust no one, not even many of their own employees.

Did Target really trust their HVAC firm or was their network just poorly segmented?

Both.

Someone hacked their HVAC firm to hack target credit swipe terminals.

At the time it was the biggest hack in US history.

Right, I'm familiar with the hack. My point is Target almost certainly didn't decide that the HVAC firm could be trusted to have access to the credit terminals - the fact that they had access was the result of poor security design, not Target's threat model.

I've often found poor security designs justified by many of the arguments in this thread that it's unreasonable to treat everything as a threat.

They know it's a bad design but doesn't matter because the threat is too improbable. Until it isn't :p

It's the everything always part of the argument that's unreasonable. You realise that that's impossible? You can't vet and control the whole stack. And, if you could, it would be prohibitively expensive.

For certain use cases, it is not cost prohibitive. Take defense or banking…

I’ve been in meetings where executives have said precisely this and I have tried to gently nudge them towards defense in depth.

Ok fair. I see the lack of simple things like segmented vlans as a lack of a threat model entirely. They trusted them implicitly, not explicitly, through their clear incompetence. Perhaps that’s better?

I think we are mostly in agreement.

Do it right, and you might mitigate threats, but do it wrong, and you are introducing more points where you could be compromised - a single supplier can be audited, a 100 cannot