
The massive consensus both then and now seems to be that the foundation of MacOS HAD to be fully replaced for it to become a truly modern operating system. Why is that—why would an attempt to introduce preemptive multitasking, multiuser support, et cetera into the Classic kernel be doomed to fail?
While I've had this question for several years, it has returned to the front of my mind upon listening to the Designed in California podcast series "Apple's MacOS Crisis" (first episode). And yes, I can and probably will ask a very similar question later about MS-DOS-based versions of Windows and their gutting and internal replacement in Windows NT.
It's especially bizarre to me that it took a Herculean, near-decade-long effort involving hundreds of people and multiple companies and development teams for Apple to modernize its cooperatively-multitasking 68k-originating operating system, involving its foundations being replaced and software only temporarily remaining compatible through a sandboxed environment, while for Atari to modernize (at least to some degree) its cooperatively-multitasking 68k-originating operating system (TOS), it took... one guy and a single year, seemingly with much more of the original OS intact† and with software compatibility largely maintained. Granted, Macintosh computers were broadly more popular than Atari ST-line computers with a larger software ecosystem, but the differences are still stark.
(I would post this to subreddits like r/AskComputerScience as well, but for hopefully understandable reasons, I'm quite afraid of the zeitgeist of the field right now...)
†Honestly not very knowledgeable about the architectural differences between TOS and MultiTOS/MiNT, so I could be wrong.