The model of Spotify in particular - paid tier alongside a free tier with ads - seems like the simplest sustainable solution I see. Having paid features is the most obvious way to make money, but you want to enable adoption as much as you can. It's the same idea as companies dangling "free trial" in front of you at every turn - in a competitive environment, you want to remove barriers for users to try your product or service. This is essentially the idea of a "loss leader" for a grocery store, or any business really.