I don't think reality is quite like your little image here. There is no absolute security, ever, but we can create layers of difficulty for attackers as approp

I don't think reality is quite like your little image here. There is no absolute security, ever, but we can create layers of difficulty for attackers as appropriate for our threat models. Someone with a reasonable amount of expertise and caution can use Linux on a personal computer in ways that make it very nearly impossible for a typical "criminal level" hacker (as opposed to nation-state level hacker) to steal information from them. Yes, that means not downloading arbitrary executables from the net, among other things, and certainly not running arbitrary code from the net like Javascript. When you do need to run something untrusted, run it isolated in a VM, etc. If you do these kinds of things, then it makes sense to also use stuff like Spectre mitigations.