The point is that they wouldn't. Only the core brew utility would, because in almost all cases, it would be immediately downgrading itself to either a dedicated homebrew user, or in the case of builds, to a nobody user who does its work in /tmp and only has write access to the one install path it has been allocated (which is then chowned to the brew user afterward).

The fundamental issue is that the POSIX user model has no concept of one user being strictly less privileged than another, and therefore all impersonations require root.

And note that the situation on Debian and Ubuntu is much worse than either of these models, because every package has an opportunity to run arbitrary commands as root as part of the postinstall and at various other times during the package lifecycle. So basically you've owned yourself the first time you add some user's random PPA and run `apt dist-upgrade` to pull new versions from it.