Brian Kernighan on the origins of Unix

If you don't want the users to communicate with each other (except over network links) then yes. You give each user their own system, whether by physical hardware or a virtual machine. My vision of a multi-user system is that it has the properties I mentioned, but at the same time you can communicate between users in the lightweight way you do on a current Linux system, provided you've granted permission. The multi-user interface provided by a set of virtual machines is pretty basic, because usually the only way for them to communicate is via TCP/IP.

We have approximations such as sandboxes used by web browsers to isolate one tab from another. Again, I see those as a kind of workaround or a hazy approximation to the right answer: creating a new user on the fly for each tab, giving it its own memory and CPU budget, and then communicating via signals, Unix domain sockets, or a shared filesystem. But yeah, talk is cheap. The point I want to make is that if it's considered a solved problem, that may be because we're too used to the limitations of the current systems. Before git, software version control might have been seen as a solved problem by some.