Something is stopping them: all the old devices that already know how to speak Spotify.

Spotify is built on bringing your music library with you everywhere. If your car or five year old stereo stops being able to play spotify songs, why would you still pay for a Premium account? 2% loss doesn't sound like much, but at Spotify's scale, that would mean loosing 2 million paying customers. If they pay $5 /month, that's not pocket change.