Subscription services have been around for a very, very, very long time now. The problem is that, like what Michael CM1 said, these people can jump off board at anytime - I did too. 10 million people AREN'T paying for Spotify, but still are using it by the way, which only makes the artist payout even more of a problem because just ad revenue isn't enough, which is what is happening to Pandora. Spotify is going to get bigger, but not in the way it wants to, free users are going to continue to add up, versus paid.
Most of us should be buying music. We're old enough to appreciate it, we have the money, and we're all owners of devices that can play it. And honestly, you should be thinking about why exactly you're paying $9.99 to listen to a playlist every month, when you could have owned that music for about that same price that you paid for Spotify in the first month. There are other means of sampling music, like YouTube, SoundCloud, and even just using the free version of Spotify.