This seems to be an area where they are limited compared to Apple.
In my view, Spotify have pretty much "won" the streaming war. They beat Apple to the market with their streaming offering, and they have a good product that people love.
The one area Apple has Spotify beat is in its understanding of the music industry. Apple's acquisition of Beats seemed ridiculous at the time, but getting Jimmy Iovine on your side in a music war is like having Goku on your side in a Universe Survival Tournament. When you've got the likes of Jimmy Iovine and Zane Lowe committed to your cause, you're going to know how record labels work, how the industry works, and given that Iovine recently came out to say that streaming services are similar and need to diversify to succeed I'd be inclined to agree with him.
Apple have failed in the past by trying to make its music exclusive and by having artists on its payroll try to attack Spotify (to backlash on both sides). It makes me think that Spotify could wrap things up by doing what you say, but licensing its own songs equally to all other streaming services. Instead of doing what Apple did, promote availability by ensuring their music is played on competing services.
Of course, to do this Spotify will need their own Iovine, and that's no mean feat. They'd need world-class talent behind them to find musicians, produce them, and market them in the way that Apple potentially could. If Spotify can build that dream team and create their own means of production then, in my view, they've won.