"Doing it any other way would be wasteful on an actual multi-user machine." On Windows at least, UWP apps are deduplicated across all users on the same system

"Doing it any other way would be wasteful on an actual multi-user machine." On Windows at least, UWP apps are deduplicated across all users on the same system so when user #2 installs the same app already installed by user #1, it will just point to the same existing shared instance of the app's files. I don't know much about MacOS but I'd guess there's a similar packaging system. These systems also incorporate sandboxes which address the other problems you mentioned in your last paragraph. The problem is just when the OS teams can't convince other developers (even ones inside the same company in Microsoft's case) to actually use their packaging and sandbox systems.