> Pre MacOS X was a 90s style OS and had major limitations that were a problem for every user, starting with no real multi-threading, and I think no protected memory.

Technically, the last “classic” Mac OS had memory protection and a fully preemptive scheduler.

The very serious caveat was that all “classic” Mac OS applications ran in a single process, and, within it, were scheduled cooperatively.

It was possible to create fully preemptively scheduled threads from within such applications, but they couldn’t write to the screen. I don’t remember whether they could do file I/O.