I watched a video on this subject a while ago; it posited that while Netflix will be able to increase their profit margins, because they pay a fixed amount for licenses (and now their own content), whereas Spotify has a pay-per-play structure. Meaning, Netflix's profit margins are flexible, while Spotify's will grow and shrink equally to their subscriber count.

Which is fine of course, as long as it's sustainable AND artists earn a fair amount. Which for Spotify is debatable, in that as an artist you can't make a living off Spotify unless you have millions of streams.