Interest has diversified (and globalization has made access to foreign music easier), and the "top" sales don't really matter any more, because between hundreds

Interest has diversified (and globalization has made access to foreign music easier), and the "top" sales don't really matter any more, because between hundreds of sub-genres, "pop" music is an artefact that doesn't represent any useful audience any longer. Teens aren't really swarming billboards, they're using social media to discover artists and… acquire music in ways that rarely impacts sales metrics.