If you look at the Copland technical docs, they describe an OS with modern underpinnings but running the Mac UI as a single process on top of it (https://en.wikipedia.org/wiki/Copland_(operating_system)#Cop...). Developers were supposed to put as much functionality as they could into background processes/threads that would benefit from preemptive multitasking and protected memory.

Amusingly, classic Mac OS ended up sort of close to that if you squint. It had a "nanokernel" that ran tasks preemptively and even supported multiple CPUs, but like with Copland the entire UI ran as a single "blue" task. The main difference from Copland as far as I can tell is that non-UI tasks were heavily restricted in the OS APIs that they could call; for unsupported APIs they had to send a message to the blue task and wait for a response. More details at http://mirror.informatimago.com/next/developer.apple.com/tec....