Premium right now is pretty lackluster, so why not make it more appealing?
I suggest that every skin is unlocked when you have premium, as soon as it runs out, you don't have the skins anymore.
That way you encourage people to keep premium and those who don't want to pay each month, can pay once to get that skin permanently.
In the end that would create a more presisting revenue for the game and people with premium would feel more encouraged to keep premium.