I can't speak to Spotify, but Netflix certainly seems sustainable with their billions in yearly profit. Either way, the average consumer doesn't care about sustainability. They just see that Netflix provides 100s of hours of fully produced TV and movies every month. Then they see for the same price they can get a few hours a month of people talking in front of microphones. There is a huge disconnect in perceived cost there. Many podcasts currently make up that difference simply from the goodwill of their audience who wants to "support" them. I'm not sure that is scalable.