It seems to me that this happens on a per-user basis as recommendations are overfitted to the recommendations they've already listened to. They overfit so much that if you put a playlist of 1000 tracks you like on shuffle, it will only play the hundred or so tracks the algorithm has decided you "really like", and it will play almost the same selection if you put it on shuffle the next day.
YouTube music (music on YouTube, not YouTube Music) works the same way but less subtly. I had a great few months discovering music through their recommendations, until literally every song I listened to would be followed on autoplay by "Sergio Mendes feat. Black Eyed Peas - Mas Que Nada" or "Funky Destination - The Inside Man (Soopasoul remix)".
It became a bit of an in-joke with us but yeah the youtube recommendation algorithm is dreadful.
IMO, Spotify is neglecting its core features, and that will ultimately hurt the platform. However, there is such a thing as network effect, and they may have the broadest selection of music. That will keep the platform growing, up to a point. Eventually, the growth numbers may peak and slow down. It's just that there's a lot of inertia right now, and the growth is still going upwards, but eventually, gravity might catch up.