You can use Apple Music on your old device or your computer as well. You do not need a new phone. So I don't see your point.
New devices = potential for new users = new free trials = potentially new subscribers.
Those who already had an apple device and used their trial are the exception. But there could potentially be a lot of new Apple users whom have never had an Apple device, thus created new Apple IDs, thus have never used a free trial, and thus could be doing so now. I didn't think I'd need to explain it out that much.