The architecture really doesn't matter, in the context of a course. Understanding concepts like scheduling, filesystems, mm etc. and perhaps tinkering with a reference implementation is more rewarding for beginners, and Minix is simple enough to allow a CS undergrad to get involved rather quickly.
So, if the architecture really doesn't matter, why not use the Lions' book and teach Sixth Edition Unix, or use "The Design and Implementation of the 4.4BSD Operating System"? Is it a holdover from the 1980s, when microkernels were still interesting, or is it because Tanenbaum is seen as a 'real' academic?
Because Minix is neither overly complex, nor obsolete. It nears the sweet spot between simplicity and functionality - I don't know how much of that I can attribute to its architecture. I don't think it has anything to do with academic credibility or holding grudges.
That's my opinion, of course, and perhaps my CS professors stuck to Tanenbaum religiously.
While Minix, like dozens of OSs, is far from having a measurable market share - especially compared to Linux/Windows/BSD/etc - is also far from being either unwanted or disused. It's actively developed and promoted. Why would you characterize it obsolete based on its difference from "mainstream" OSs? That doesn't make sense.
