"Isolation and specialization can be effective defense concepts."

Thank you - appreciated.

Many people (unwittingly ?) argue against defense-in-depth because they look at individual layers of the defense and declare them inadequate. They are always correct.

What they are missing is that nobody ever proposed only using (port knocking, or stack obfuscation, or fake login banners, or whatever). They are always additive layers of security on top of the existing set of best practices.