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OS-level sandboxes are way too coarse grained to achieve a good "hollowing out" of the attack surface. The principle of least privilege should extend down to/start at the individual language library level (because this is where the actual trust boundaries are), or even finer grained, at the individual function or code segment level (thereby providing maximum control), and therefore not be limited to larger domains.

Most software today relies on many (imported, third party) libraries, so the security architecture should provide primitives/abstractions to manage rights at that level, which requires programming languages to implement the ability to sandbox (managing the effects of) code. If they did this with lightweight, portable virtual machines like WebAssembly, that could work.

The vast majority of code out there should be limited to pure computation and have no ability to access anything external at all (and otherwise, only what it actually requires) - yet most languages are simply incapable of providing any such guarantees. If the programmer of software cannot get ironclad assurances, they cannot in turn provide them to their users.

I'm not saying that OS-level sandboxing isn't good, just that it doesn't go far enough. And depending on the setup, it may not sufficiently limit the effects of compromised elements, and it provides no "monitoring in the small". It's also not convenient or efficient to have an entire OS instance for every single system component. Compartmented microkernel operating systems like Genode do it better imo.