We might discussing this from different views, network segregation is important even if you build everything as zero trust, firewalling something it's one of the easiest way to not trust it.

I think the difference in perspective is that I only really need to work in very narrow an well defined interface to systems that can be hacked, so my view of what the attack surface is affacted by that, I do not care about. For me as long as revealing my password to my client is fine I'm fine. My client can be full of malware without affecting security severly.

Clean room; "leave everything connected on the outside", and variants of that is REALLY ineffective. I've measured that the waste from that, back then we spent alot of time classifing what software had to be developed in such an environment. You do not want to work that way.