I don't think the fact that people know what to expect when you say 'experimental' indicates that it's no longer experimental. I think it has more to do with the fact that you reach a point where the more you fuck around with music the more it sounds like random noise. There is a handy analogy to make (well, it's not an analogy really, since it is probably still part of the avant garde, right?) to extremely atonal or serialistic music - although you might write two pieces completely differently from different starting points using completely different methods, the brain generally can't discern the patterns written into it, and so they end up sounding more or less the same to the casual listener who doesn't follow it with a score. Eventually you may as well cut to the chase and get a signal generator to create random white noise.
Or, for an analogy to visual art, if you keep adding more and more colours to a painting, you will always generally end up with some horrible generic brown. I guess, at least harmonically, if you keep adding notes you obviously lose the harmonic structure, and so from a certain point the brain is no longer able to discern anything besides a wash of noise; at this point everything sounds the same. After a while it's not really surprising that people would prefer that the artist just stuck to using one colour at a time, because the pictures used to be a lot more interesting.