I doubt very much google really cares about Play Music sales, it likely only has it because it wants to offer comparable services to apple in order that they don't switch away from android. By far and away Googles most important revenue stream is advertising, youtube fits into the advertising model far better than Play.

Ultimately a generation of kids are growing up right now who use Youtube as their primary source of entertainment and for whom downloading an mp3 is likely to seem archaic. I doubt they're going to switch to Spotify so long as google can figure out offline play for audio (which I've no doubt they can)