You're right about it being sound design rather than music. It is striking how at the same time that this is happening in film music, there are various programs coming out that automatically generate a music score, even one for consumer home videos from Roxio!
I don't think it is because composers are worse today, but they are being attracted to this kind of sound because it is successful in contemporary scoring. It is an exploitation of a simple fact of film music - that a single sound or series of sounds without any real musical value CAN be used to underscore a scene. Not as well as real music, but in a basic way. Some composers who are not capable of doing real music exploited this fact, and producers have grown to accept that sound as real film music. Maybe Zimmer can write a great symphony on the side, I don't know, but his film scores don't show it. It takes a full blown genius like John Williams to do the type of scoring he does and he is ultra expensive so perhaps that contributes to this shift as well. Also, Bernard Herrmann was famous for being incredibly obnoxious. Probably producers don't want to deal with the hassle of a great composer, and prefer a slick pro guy who just comes in and provides an efficient, predictable, useable sound for their film.
Though again, part of what bugged me about this particular example of Christopher Nolan and Zimmer is that it now seems that they are forming a duo. But if you look at the great director-composer duos of the past - Herrmann-Hitchcock (the ultimate), Truffaut-Delerue, Tim Burton-Danny Elfman, or even Spielberg-Williams (though Spielberg is nowhere near Hitchcock or Truffaut) none of those composers are the lowest common denominator, minimal type like Zimmer and are maniacally serious about creating a richly detailed musical work. I actually enjoy listening to the music of those guys as much as any concert music! Oh well, this is just my wondering what the hell is going on. I don't know what to think about it...