I would really like to have my playlist persistent, no matter what happens. When an artist (like Taylor Swift recently) decides to leave Spotify, I would just like to replace the songs with local files. But currently they just disappear. I don't even get a message which songs have been removed from my playlist. So at the moment I have to be constantly "afraid" that from one day to another a song disappears and I don't even realize it.
So please just give me a message when a song was removed and let me replace it with local files. And make playlists persistent and/or let me save them locally (not the songs, just the playlist!).
As Carina_ mentioned, a similar idea has also been suggested here:
Add your kudos and comments there please!
You might also want to go into Preferences -> Playback and uncheck 'Hide Unavailable Tracks', so that you can see which tracks have been taken down from the Spotify library.