This is pretty interesting. I used a couple of well-known systems of the late 80s to early 90s that had cooperative multitasking and no memory protection. They were certainly usable, and people did real things with them, but I definitely remember hitting the limits. The main thing that hit me with Windows 3.x (on a 16MHz 386) was that a background terminal program could not process interrupts fast enough to keep a 2400 baud Xmodem transfer from crashing while I was playing Minesweeper or Solitaire, or using Word Perfect in the Foreground. I remember Mac IIs being a little better with that sort of thing, but also very subject to user programs bringing the whole system down.