That's not my argument. My argument is this: a reason Spotify might have decided to move into podcasts is that podcasts are more efficient at generating content

That's not my argument. My argument is this: a reason Spotify might have decided to move into podcasts is that podcasts are more efficient at generating content (measured in hours) than musicians are. It doesn't go the other way though; there's little incentive to generate content less efficiently. (Ignoring the fact that Amazon/Audible do in fact distribute music through Prime Music.)