Good points, I would say that defense in depth is useful when the layers of defense need to all be broken (more or less) independently for a successful attack (

Good points, I would say that defense in depth is useful when the layers of defense need to all be broken (more or less) independently for a successful attack (this fails only if you add layers that expose vulnerabities compromising your system). E.g. usually a sandbox satisfies this criterion. Also, whenever some layers may allow compromising everything, the supply chains of the layers should be minimal or correlated (same supplier), to avoid increasing such supply chain risks.