Sure, but framed that way you also need to be able to run programs that think they have higher permissions even though API calls are returning mocked/sanitized

Sure, but framed that way you also need to be able to run programs that think they have higher permissions even though API calls are returning mocked/sanitized data. And more generally, the ability to run programs with high permissions that can completely modify the behavior of other lower-permissions programs (eg HSBC).