as a person who has easily spent over $100 on skins for this game.. i would seriously consider quitting if riot decided to add a monthly fee for ANYTHING
league has also been free to play, and money doesn't give you any advantage. (well except rune pages, but that's stupid and an argument for another time).
this is the BEST type of game.. you play it because its FUN, and you spend money only for visuals, not because its makes the character better. but because its something you like. restricting a game may to a payed format would not only be horrible.. it would pretty much be again what riot has made he game to be "free to play"