Amazon actually announced their service was closing down last week:
https://developer.amazon.com/blogs/appstore/post/cbadeae1-99...
I would be interested in seeing data about how successful that program was at attracting actual consumers to the platform rather than just developers . . . .
In regard to your other points I also happened across this today: http://www.informationisbeautiful.net/visualizations/spotify...
Interesting how quickly these subscription services are losing money and how low the rate of return to the artist is.