Spotify sure is trying to turn podcasts into a subscription service. They buy as many as possible and add them to their subscription model is their plan. They at least get ad revenue if people don't move off their app.

Luckily the landscape of podcasts will not allow them to do that. There are way too many podcasts that are popular and make money. So buying all of them will be impossible. In a way they are similar to the open source landscape. Sure, there exist Apache, mysql etc but the ones that are not popular are equally important for the medium.