I think more important than the P2P network shutting down is how it allowed Spotify to get up and running with relatively low up-front investment. What they do after this is, ultimately, unimportant and not terribly interesting.
As their service grows, Spotify should be liking P2P superlinearly more. Since they and other services are apparently not, it something is wrong with my conception of the economics of the internet.
Only if you assume that the price of bandwidth is linear. It is suggested in other comments that when you buy bandwidth in bulk you can get large discounts. If I don't misremember, back in the early days of Youtube, they came to a point where the network operators wanted to get access to Youtube rather than the other way around.
Also, as a company grows and get access to more capital they might be able to invest in other infrastructure that is needed (other than just bandwidth) that combined with the bulk prices, make the opportunity cost of developing the p2p system larger than the opportunity gains.