I've read posts that people are muting the available music and putting on their own stuff. At that point, your limitations of original, constant BPM music clearly don't matter to some people. I understand there are copyright limitations of using non-original content, however if you're just allowing the song to play through your app and not fitting it to a workout, then at that point it's the same as any other playback app. Please consider as this will make doing your workouts much more engaging, which means more subscriptions and less people leaving :-)