Even though there were some benefits to the modularity of Multics (apparently you could unload and replace hardware in Multics servers without reboot, which was unheard of at the time), it was also its downfall. Multics was eventually deemed over-engineered and too difficult to work with. It couldn't evolve fast enough with the changing technological landscape. Bell Labs' conclusion after the project was shelved was that OSs were too costly and too difficult to design. They told engineers that no one should work on OSs.
Ken Thompson wanted a modern OS so he disregarded these instructions. He used some of the expertise he gained while working on Multics and wrote Unix for himself (in three weeks, in assembly). People started looking over Thompson's shoulder being like "Hey what OS are you using there, can I get a copy?" and the rest is history.
Brian Kernighan described Unix as "one of" whatever Multics was "multiple of". Linux eventually adopted a similar architecture.
If you click on the link, I mention other competing attempts and architectures, like Multics, Hurd, MacOS and even early Windows that either failed or started adopting Unix patterns.
Unix was an effort to take Multics, an operating system that had gotten too modular, and integrate the good parts into a more unified whole (book recommendation: https://www.amazon.com/UNIX-History-Memoir-Brian-Kernighan/d...).
Even though there were some benefits to the modularity of Multics (apparently you could unload and replace hardware in Multics servers without reboot, which was unheard of at the time), it was also its downfall. Multics was eventually deemed over-engineered and too difficult to work with. It couldn't evolve fast enough with the changing technological landscape. Bell Labs' conclusion after the project was shelved was that OSs were too costly and too difficult to design. They told engineers that no one should work on OSs.
Ken Thompson wanted a modern OS so he disregarded these instructions. He used some of the expertise he gained while working on Multics and wrote Unix for himself (in three weeks, in assembly). People started looking over Thompson's shoulder being like "Hey what OS are you using there, can I get a copy?" and the rest is history.
Brian Kernighan described Unix as "one of" whatever Multics was "multiple of". Linux eventually adopted a similar architecture.
More here: https://benoitessiambre.com/integration.html