The iMessage domination is automatic -- if you message from an Apple device to a phone number that happens to be an Apple device (which the sender might be oblivious to), it's an iMessage. People aren't subscribing to iMessage, or intentionally choosing it, the thing is just the default in that case.

Apple Music heavily pushed, on the device, a 3 month free trial, and it's about time that most of those 3 month trials expire. I imagine the number will drop substantially after.