I obviously do understand why people use Spotify.

But you are correct; my phone has 128GB internal storage, my hard drive music collection is smaller than that. There has been solutions for syncing (a curated list of) music to mobile devices for a long time. There are many - and frankly much better - ways to discover new music outside of Spotify. There are some really cool music only internet radio stations out there.
And I'm not even talking about self-hosting, which simply cannot be recommended to everyone.

People defending Spotify always come back to convenience, and always dismiss the disdvantages of generated playlists, and downplay the suckness of AI content. And never acknowledge that alternatives exist, right down to NOT using a music streaming service.