We only think of Unix programs as simple because we have many more abstractions nowadays. But you should compare a Unix program with DOS programs (probably CP/M also but I never wrote those myself) at the time. Poking directly at the hardware, using segmented memory, dealing with interrupts. The idea that a program should be well behaved, should accept things an inputs, should push outputs, and should have a virtual address space are actually huge abstractions over what could just be a block of code run on a spare OS. I'm not saying that microservices are better than monoliths, just that Unix programs aren't as simple as we think they are in a world where we're managing servers like cattle and not like pets.