> However, that same reason doesn't apply within a single app, because a single app by a single author _can_ safely co-operate with itself. So co-operative time

> However, that same reason doesn't apply within a single app, because a single app by a single author _can_ safely co-operate with itself. So co-operative time sharing can work and make sense within single app. That's not the case in any non-trivial app. Any real program truly has dozens if not hundreds of authors whose code you're using sight unseen, and which may be difficult to modify. And your program getting stuck can be just as bad as the OS getting stuck. Suddenly your program doesn't reply to API requests, or doesn't relinquish some expensive resource (like an expensive VM), or some such.