Few certainties, mostly because new songs and "sounds" are decreasingly innovative, so the inevitable trend is fewer songs of permanent value. Fewer songs will stick out for more than the six months or a year they're in mainstream rotation. More disposable music will come and go in very short time periods, but the least disposable of the people making the disposable music will still be around in some form: Kanye West, Lady Gaga, Jay-Z, Eminem, Arcade Fire, etc. Their best songs will probably still be getting airplay twenty years from now, and not because those songs are actually great or anything. I bet "Paper Planes" is around a while too. It might be the "Bittersweet Symphony" of the 00's.