Windows 3.1, for example. It had a very elegant Virtual Machine Manager with pre-emptive multitasking and true process isolation... and then GUI programs were all placed into a single cooperatively-multitasked event loop, because anything else would have killed performance. And the OS also had to continue to support IO device drivers written for DOS—that could disable interrupts system-wide and thus freeze the computer!—because otherwise your (horrible, un-QAed) scanner wouldn't work.

I fully believe that software hasn't gotten much better over the last 40 years. The hardware constraints have simply relaxed to the point that we're no longer forced to make as many Faustian bargains.