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They already do this, they limit which streaming services have their content, and do time/exclusive releases for specific services. Apple bought Beats for the app and the user base, it's not impossible to already do promotion like some headphones give you 20$ worth of iTunes credit, now it's even easier get Beats and get access to additional exclusive content, or like some one else suggested better quality content. Seems like a good way of pushing people to pay 300$ for 30$ worth of hardware, or more precisely helping people to justify their purchase because their headphones can actually now sound better than even entry quality studio/recording headphones.

DRM can rarely be used as a protection against mass copying and pretty much never when you cannot control the analog output of the protected content, it's only real use it to restrict how lawfull consumers can access and use the content. External DAC's have existed for Android devices for quite a while (BT/USB DAC's), and even for iOS (you could connect a DAC to an iPhone/iPad since they switched to the Lighting cable). The streaming industry has almost hit a dead end with monetization, having a digital interface that can be controlled gives them a whole new avenue of value added services and while it's unlikely that they'll jump on it immediately I would bet my money on the likes of Spotify, Amazon Music, and Apple Music capitalizing on this within the next 2 years.

Does it makes sense for an artist to make their music available to only one streaming service? Some would say no, and yet they still do it.

...No, they make money by people paying to listen to (or play) their songs.