I'd stick with always. Defense vs offense in anything reasonably complex suffers from one issue that simply cannot be overcome. To defend, you need to defend against every single possible imaginable attack, from now until forever. To attack, you need to find a single attack that works. And on a practical level all systems need to be accessible by somebody, yet that somebody is himself also now a part of your security structure and is never going to be 100% reliable, both in terms of corruption and incompetence.

> not all systems are vulnerable to undervoltage attacks. For example, I don't think the iphone secure enclave is vulnerable to this.

Then there's decapping / depotting, a world of different types of microscopy - some destructive some not, directed EM attacks, etc.

> And good security uses "defence in depth"

And automation has enabled "offense in depth"

> To hack chrome, you need a vulnerability in the renderer or VM. Then you also need a sandbox escape, and a way to use that to attack the browser's parent process.

Or you just phish the user into installing your exploit. There's always another layer. Always a potential exploit. Because ultimately the same properties of the universe which permit computation within a closed system allow for predictably observing and influencing it. The expense and hassle of doing so are widely variable, of course.