I generally agree with engagement being a BS metric, I'm not sure I agree in this case, unless we separate out time I'm actively interacting with the app and time I'm listening to music and only call the former BS. When I first switched to Spotify from Google Play, I listened to a lot more music because the recommendation algorithm pointed me at a ton of music I ended up looking a lot. Now its recommendations have gotten to samey and my engagement declined. I'd love for them to improve recommendations and drive my engagement up that way.

Now, similar example is Netflix. I've at times reached a point where I think I had watched everything on the platform I'd enjoy. Their UI keeps showing old stuff as new tough, different pictures for the same show, etc. This led to wasted time and frustrating engagement. (Aside: the best Netflix UI ever was a script I wrote myself when they still had a public API that just listed all movies ordered by how much they predicted I'd like it. That paired with IMDB ratings and filtered by what I've already have seen would be the holy Grail but it would be obvious when it's time to unsubscribe for a while)