MVS/OS390/z/OS was designed from the very beginning to be a multi-user multi-tasking environment.
MVS has the BEST task controller and dispatcher of any operating system in the world, zero wait states, 5 bubble levels of task abend control (and that's not counting the hardware error controls).
My machine has one gig of CS, 400 gig of disk, and comfortably supports over 100 concurrent users.
And it only runs at about 40 megahertz. The really big ones run at 40,000 megahertz.
Yeah...
While I agree with your basic premise, most business have neither the need or the budget for big iron.