I don't have the time to post an enormous response, but this (brilliant) essay by Glenn Gould goes well in the context of the discussion being held:
http://kunsthistorie.kunstakademiet.dk/mortlake/pdf/2011/gould1994a.pdf
There are many ways to imitate while incorporating something which did not exist in the original (assuming one wishes to do this) but which could still maintain a high degree of similarity on some level or another. You could, theoretically, write a sonata with five themes which presented them in reverse order in the recapitulation, thereby creating something of an arch structure, but which was as diatonic as anything in the Classical era. You pointed to Brahms as one example, but I think it's clear that some people on this board would have been among his detractors if they had lived in his time.
Music (that is, instrumental music) is more abstract than the other arts, which is why it can get...more of a free pass. People ascribe all sorts of historical meaning to this and that, but when I listen to some great concerto or symphony by Mozart or Beethoven, I don't have images of wigged aristocrats or burgeoning bourgeoisie in my head. I do, however, get more of that impression when I listen to the lesser composers of the times.