I ran Minix for a year or more on my Atari ST - having a UNIX-like operating system on a machine I could have at home was a truly awesome thing. Tanenbaum's work is fascinating, useful and will be around for a good while...which is more or less the definition of "successful" in academic circles.
The debates with Linus were interesting - but I always felt that they were arguing at cross-purposes. Linus wanted a quick implementation of something indistinguishable from "real UNIX" - Tanenbaum wanted something beautiful and elegant. Both got what they wanted - there was (and continues to be) no reason why they can't both continue to exist and be useful.
Tanenbaum's statement that the computer would mostly be running one program at a time was clearly unreasonable for a PC - but think about phones or embedded controllers like BeagleBone and Raspberry Pi? Perhaps Minix is a better solution in those kinds of applications?