I personally cannot imagine having my whole collection depend on a service like this. I use musical streaming for discovery (on free plans), and the actual collection is all local.
Also I use Yandex Music because its recommendations just work for me. Tried Spotify several times but the algorithm just was consistently not getting it. Not to mention it wanted you to enable DRM and was frustratingly slow compared to other applications I use.
Not needing a dedicated mp3 player
Well, and Spotify doesn’t even give you such an option on the other hand. Only desktop and smartphone, probably wouldn’t allow one that is perma-offline, and only in the official proprietary app/webapp.