The projection graph does assume flat physical sales. But it's a stacked chart.

Paying to paying is not completely unreasonable, but imagine an airline using the average fares of its Airline Club members (who are often flying first class) vs the fares of average Americans. It's easy to inflate the number when the group of people you're talking about are rich and predisposed to being more engaged.

edit to your edit: It's not clear to me why you would be willing to listen to Spotify for free, but then buy music on the side. Anecdotally, I've never met anyone who does that consistently. I'd be interested to hear contradicting anecdotes though.