The crowns of loyalty are a very good looking item in game but, they have no practical use. This I why I am suggesting that you receive the actual crown of loyalty AS WELL as a cosmetic override of the crown in your wardrobe. I hope you take this into consideration as I've seen it done for outfits such as the desert treasure outfits (Damis light and dark, Fareed, and Kamil). I look forward to hearing back from someone about this or just seeing it added into the game.
While a keepsake key is currently an option, id rather it just be updated to a wardrobe cosmetic. I see no reason to buy a key as I am tired of everything being based around micro transactions. I understand that they are a company and they're here to make money, but people like you suggesting to just buy everything rather than requesting a simple update are the reason Runescape has become what it is today. Back to my original point, it is a cosmetic and it should be a Cosmetic override without purchase of a keepsake key.
Draco Burnzsaid: Just use a keepsake key. You pay membership to gain loyalty rewards, so it really only would make sense if they make it an override for "free" - forcing you to use a keepsake key is double dipping.
I'm sure the reason for it is because these crowns were made long before we had cosmetic overrides and they never wanted to assign someone the task of updating already developed content.