We are at an interesting time, after the extensive investigation, exploration, and innovation in music of the past 100 years. There may be fewer, obvious, new directions to discover. Concurrently, almost anyone can participate, record, and disseminate. The venues for exposure have increased many fold, but many of these venues are impersonal and depend on the chance someone will stumble across them. It will be harder for an Elvis or a Beatles to emerge.
OK, now at the risk of being black-balled from the forum, I think a philosophical argument can be made that the evolution of Modern [in the construct of Modernity] music culminated with the Beatles. Music after the Beatles, the same as Art after Andy Warhol, is different and can be considered Post-modern( ref. The End of Art, Kuspit, and After The End Of Art, Danto). If you study Contemporary Art, you will see a correlation between the paths all the arts have followed since the mid-sixties. I am not saying the Beatles were the best band in history, rather that the aesthetic they brought to the music, and the impact it had on the music industry, the arts, fashion, and culture was an event that changed everything.