Kind of a bs article. Valuation increases in companies vs artist income doesn't directly make that much sense. Not saying artists aren't being shorted simply this sort of analysis is not telling much about why or what might be fair.
Some hold once music is readily available and easily copied it becomes worth nothing. You have an infinite supply for a limited if large demand. Spotify's value is in its abilty to supply this music to consumers. A real cynic could even hold Spotify has improved the number of ways, the ease, the covenience, and volume of streaming possible for its customers which adds value justifying its increase in valuation. While musicians have contributed nothing to these values to the consumer. The music was already there.
I agree it is a broken system for musicians.