For the record, sandboxed macOS apps do exist (as someone mentioned upthread) and have a design somewhat similar to what you're describing, but uglier for the s

For the record, sandboxed macOS apps do exist (as someone mentioned upthread) and have a design somewhat similar to what you're describing, but uglier for the sake of backwards compatibility. For each app you have a directory like /Users/foo/Library/Containers/com.some.bundleid/Data, which is an entire virtualized home directory, containing not just a Library subdirectory but also Desktop, Documents, Downloads, etc. The latter directories shouldn't actually be used, but they're there in case some legacy code tries to access them. When an app presents an open or save dialog, the dialog is out-of-process and unsandboxed, so the user can pick a file from their real home directory or anywhere else; once they do so, the app is automatically granted the ability to access that particular file.