alt Hacker NewsI get that multiple human users on a same machine is rare nowadays, and that per-app users were never a thing.
But windows still has a root and a lower privilege user. You typically need to click on "run as admin" to elevate privileges to, for example, alter system binaries.
I know that Chrome on Windows tries to lower its privileges to mitigate exploits, and although it's not very popular, the MS Store app platform does try to do full isolation of apps. So actually, per-app separation of users kinda does happen, or is attempted on Windows.