The phenomenon of minimalist music I think is better understood from its initial stages, for after market popularity sets in the theoretical fascinations seem to dissipate, seem to become rationalized away as unimportant and cumbersome. If you take all four praticioners here, to my mind only their early works seem to hold any interest at all,it is only the only works that have a "longevity" factor, where we can still find points of interests. For it was in the early works that carried the weight into what we have now.
Musical minimalism as well is a kind of misnomer in that the term began in the visual arts and if you go there you will find the term and its results and achievments has a much more vigorous base of contemplation and export. There simply is more important things happening there, as Donald Judd,the minimalist shrine of cubes and geometric shapes in an old Army base in Marfa Texas or the flourescent lighting schemes of Dan Flavin, the powerful sculptural plates of Richard Serra,or painters abound as Ellsworth Kelly, Frank Stella and Bridget Riley. There simply is no comparison with the level of conceptual depth and gestural focus, what art is suppose to do, what it did, and how the concept is engaged,and how it responds to its context and art history,or a temporality (how for instance the spirit, ir-religious of course is engaged in Richard Serra, his plates where the human mind simply stands there engaged in peace with his own existence or sense of space and time). Musical minimalism has no equivalent, and it is a shame for it could have had this. La Monte Young's "Well-Tuned Piano", a 9 hour work with just intonation tunings of the piano comes close to the temporal vigours of Judd's shrine I beleive.
With the introduction of opera in the works of Glass well now we are in another dimension, for Glass resorted to traditional classical structures of opera, duets, trios, quartets, (as in Aknathen) this is no longer innovative means. Potter's book draws light on this paradigm here makes you think of these issues what minimalism did and what it is now. Was it simply a fad?, or did it produce sustainable music?, music we can return to once or twice, or was minimalism simply "grist" for the mill of the market, one time, make the cash, take the money and run. Again the importance of minimalism is found in its early repertoire, Reich fascinating threadbare music for four woodblocks was all he needed to write to proclaim a status, or Glass's early music with Farfisa organs and saxophones,"Music in Fifths" or Riley's "In C", or his "Keyboard Studies" are all relevant pieces we can return to unpretenciously.
The late Morton Feldman is of course not here. He had intense knowledge of the visual arts world and his last works sought to reclaim this paradigm for music (his Second String Quartet, and Triadic Memories, For Chritian Wolff) are works scaling long durational lengths a place where minimalism in music seems to be now a beginning point not an end. Had Feldman lived I think he would have written even longer works. Of course Cage's massive work for organ now being realized in Halberstadt Germany, a work lasting years is also a step in the right direction.