Well I can’t speak for a whole country but melon been on the decline, and considered how this is literal criminal activity I can only assume it will put in more of a decline
The only way spotify can succeed in south korea is if they manage to get better licensing for korean artists (korean people prefer to listen to korean artists over western artists) and they make their subscription cheaper than domestic platforms (currently it's more expensive). take a look at what happened to apple music.
For kpop fandoms especially, spotify would need to be added to instiz and/or some kind of annual award show - otherwise fandoms will still prefer to stream on domestic platforms for the RAK/CAK/PAK + annual awards.
at the same time, this forum's over-importance given to melon is a little misplaced. since melon's market share is declining, it's much better to look at overall ichart position (or at least combination melon/flo/genie daily charts) for domestic song performance.