> The ones that wanted to work on Linux could just do that.
Sure, but time is a very finite quantity and wasting a couple of years on Tanenbaum's pet project may have resulted in some residual knowledge about how operating systems in general worked but looking at most of the developments they pursued the bulk were such dead-ends that even outside of VU there was relatively little adoption. The world had moved to Linux and VU refused to move with it.
From being ahead they ended up being behind.
I wonder who you are thinking of who 'wasted a couple of years'. Regular students do one course in operating systems. That is a series of lectures and some practical work. The practical work is a couple of weeks at most if you know what you are doing.
Some people spent a lot more time on MINIX, but that was either as a hobby or the PhD students who worked on MINIX3. But MINIX3 generated lots of papers with a best paper award, so that can hardly be seen as wasted from an academic point of view.
I have some friends that went that route. They did not come away with anything that helped their careers later on and the 'academic point of view' in CS in NL hasn't been the best way to put food on your table since the days of Dijkstra.