It could easily stop including those if the record companies think Spotify is competing with them instead of helping them. I think that's the point being made: that back catalog is leverage that will stop Spotify from ever rocking the boat the way Netflix has.
They aren't saying Spotify doesn't _currently_ have those artists, they're saying that they wouldn't survive as a music streaming service _without_ them, as no one only listens to the latest and greatest, as many do with TV/Movies.
Thus, Spotify can't directly attempt to go in the same direction that Netflix did, as the other labels would see them attempting to compete and could possibly pull out, destroying any reason to use Spotify in the first place.
Last I checked, Spotify has those artists. Have you actually used Spotify lately?