The impression I get (university class on OSes) is that the microkernel/modular approach is more about making it simpler to port than making a specific instance simple? Similarly it'd be less complicated to extend since that's just a case of adding a new loadable module. In terms of a learning OS I'm not sure exactly which should be prioritised - extendability/portability or a simple structure? If this is a topic with a lot of depth I'm obviously not getting I'd definitely be interested in a discussion - my exam is soon :)