The begged question that mainstream music quality has declined is one of the more curious things I have read in quite some time, to say nothing of the implication that the same will happen with, say, dramatic content.

You may not like, say, Miley Cyrus (I don't), but artists and groups like Daft Punk and Radiohead are quite mainstream, quite successful, and quite good as well. The broadening of what is "mainstream" has added a lot of choice for pretty much everyone and a lot of it is quite good.