Just like with anything digital you (and Spotify) are fully at the mercy of the rights holders. When (not if) they pull their stuff, or replace their stuff, or change their stuff, you can never get the original back unless you preserve it.

Largest example: a lot of Russian music is not available on Spotify because of the Russia-Ukrane war, and Spotify pulling out of Russia. So they don't have the licneses to a lot of stuff because that belongs to companies operating within Russia.