there's nothing stopping you from having both!
I havnt done this yet but im thinking about cancelling my spotify subscription soon and only signing up for a few months out the year just to discover new stuff, then put the rest of the money I would have given spotify that year towards buying the music I want to keep.
I won't have to worry about songs disappearing anymore or my playlists not being downloaded offline and more importantly you are giving an artist some money and not just a fraction of a cent