I'm cleaning out my gmail folders this weekend, and went deep into the archive to 2011, when I got my invite to Google Music.
It's funny, because I just (November) moved all of my music out of cloud and back to local-only. Amazon was the last straw, when I tried to play purchased music, and was forced to listen to it on shuffle with other songs not of my choosing.
Anyway... there was a time when Google (ahem, Youtube) Music was set to be a game-changer. Imagine if enshittification wasn't a thing.
Oh yes, the enshittification of music streaming.
To be fair, most people seem to be fine with a "broadcast radio" type playback, where they want to hear both the music they already know/like, with other music that is at least somewhat similar to it.
A nontrivial number of people are more like you, who want this specific music to be played and nothing more. But that perspective, at least from what I've seen, is not held by the majority of users. So we get random trash thrown in with our personally curated lists of songs and albums.
Catering to the majority is fine, IMO, since that's what will pay the bills. I get it. From a business perspective it makes sense. However, ignoring literally everyone else in the process is not what I would consider to be an acceptable policy. Certainly make the defaults conform to what appeals to the largest number of people, but allow the individual user to customize their experience.
Since companies won't do that, those that want to listen to specific music generally get pushed into having a local music collection, so it behaves in a way that makes sense.
I've never had Spotify put anything into my curated playlists. They only play the music on the playlist.