> Apple could have worked in multitasking in classic MacOS if they really wanted to
They did, and they couldn’t. Most users had some code running that patched system calls locally or globally or that peeked into various system data structures, and all applications assumed the system used cooperative multitasking. Going from there to a system with preemptive multitasking would mean breaking a lot of code, or a Herculean effort to (try to) hack around all issues that it caused with existing applications. I think that would have slowed down the system so much that it wasn’t worthwhile making the effort.
Having said that, MacOS 9 had a preemptive multitasking kernel. It ran all ‘normal’ Mac applications cooperatively in a single address space, though. Applications could run ’tasks’ preemptively, but those tasks couldn’t do GUI stuff (https://developer.apple.com/library/archive/documentation/Ca...)