People can argue about what is folk music till the cows come home, and that's because the phrase 'folk music' is not an intelligent phrase.    Music isn't described by who wrote it or who sang it. It is described by its rhythm, melody and harmony. And nobles and peasants, scholars and the illiterate can use any kind of rhythm, melody or harmony.

To take a parallel instance, the Jeep was invented to drive in rough country during World War II. We could have called it a war vehicle. But now jeeps are used by suburbanites and hunters, back-country nurses and paleontologists. It's not a war vehicle anymore, and now we realize that 'war vehicle' like 'folk music,' is not an intelligent name.