Mostly because it strikes me as deeply illusory. It ignores enormous cultural changes that arise in even very homogenous cultures that stress continuity and stability. Realistically, whatever is being praised as "classic" or "traditional" probably only existed transiently. If change happens regardless of the demographic pattern, then is it really sensible to use the latter as a stand-in for the former? Likewise, is there anything that precludes things from retaining value even as they are introduced to new peoples? In light of all this, the mealy-mouth talked of "demographics" seems little more than coded language for disliking other ethnicities because they are not one's own.