Have a button somewhere that lets you switch to a different user. When you switch, you enter that other user's PIN or password. Then you see their apps, have their data, etc. When you switch back, you see your apps, have your data, etc.
It doesn't have to be very complicated. Even on a Mac, the "cruft" amounts to my name sitting in the menu bar, and having to choose who to log in as before I put in my password.
First, all my system settings wasn't cloned, to the new account i had to re-add things you take for granted like my localized keyboards and stuff like that, can't remember if wifi passwords were included or not. Then comes application settings, many apps are unusable before you configure them properly, even the web browser is unusable in the default state on high dpi devices, you must change the default zoom level. Then it seems some apps actually share state, and in some cases rightfully so, finding out how they share it is a mystery. Some are designed with multi user in mind so you can in fact have a mail app accessible on both accounts without sharing your mails as the system accounts are linked to different mail accounts. Others are the other way around, maybe you want your 10GB music collection to be accessible from every account? Drawing the line of where and what should be shared is very difficult.
It's not gonna happen.
I see no need to overcomplicate it. A user account system where each account acts like a separate device (aside from the unfortunate realities of sharing storage resources) would be fine. What apps share state? None! What music gets shared? None! What settings get shared? None!
When does something cross the line from "system wide driver/configuration" to "user specific data/configuration"? If absolutely everything should act like a separate device you might as well dual boot or use virtual machines.
One of my scenarios for shared data is that when I'm working i want to listen to my music, but when i have a party i want to use the same device as a jukebox with the same music but not open up access to my documents. I know that i'm not alone with this problem. Trusting your friends to not dig around is another topic but with pop up notifications and active widgets on the desktop even your most trusted friends might get private emails shoved in their face even if they are trying to avoid them.
The idea that keyboard localization should be system-wide is weird. Different people can speak and write different languages. Although it should be a non-issue, since a single person can speak and write multiple languages, and switching keyboards should be trivial.
Anyway, you can come up with scenarios where my "make it act like a fresh device" idea is inadequate. But my idea is still an absolute improvement over what we have now, and would be perfectly good for a lot of uses. You're describing a bunch of stuff you want but that is by no means required, and then using the complexity of that stuff to say that the whole feature is a bad idea. It doesn't make a lot of sense.
