Now you pay Spotify $10/month for unlimited access to the entire album. To the entirety of the artist's catalogue. To the entirety of all the included artists' catalogues.
This is obviously and trivially less money than any one of those artists would make previously from you if you liked their music.
What makes you so sure there's less money here?
I remember that we used to pay about $10 per album (with the exception of certain top 40 new releases that cost twice as much that I never bought), and I used to buy about 1 album per month. If everyone who did that switched to Spotify for the entirety of their music consumption, that's exactly the same revenue going into the system as before.
It's even better now. Under the old system, if you liked an artists music you bought it once, and that was the end of the transaction. Especially for new artists with only one or two albums, that's tough. Who goes out and buys an artists' entire back catalog, anyway? Under the new system, if you like the artists music they can keep getting paid as long as you keep listening to it.