Sure, but that's mostly academic: compromise of the user account is game over for any real user. Not actually being Administrator isn't much consolation when the regular user account can extract your cookie jar, record all of your keystrokes and mouse movements, record all desktop video (except for DRM-protected content, heh) etc.

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.