Can your grandma use SELinux? Delusional.
Normal people's use cases for their computer is light file management, light document and productivity workflows and everything else is done in the browser. Hell, most of the document processing and productivity crap is in the browser these days too.
That's rather self critical of you, even if deserved.
My grandma also can't write software, or really do anything advanced, no should she be able to. SELinux, just like any other security and/or containerization technology, is supposed to be used by developers, sysadmins, distribution maintainers. Not by end users.
Is the macos sandbox the odd one out? I'm not familiar with it, but find it very hard to believe that "my grandma" is its target audience.