While yes the active League players may increase this could be seen as harmful to riots vision. They heavily want the competitive 5v5 scene to flourish. That's part of the reason they have these gamemodes. They break the monotony for some players to keep it fresh so they play for longer, along with other players firing up league after a lengthy hiatus so they play the game mode only to start enjoying the game once again and get back into the ranked scene.
As for if they were to ever put out a different game mode all the time, this would be counter intuitive to almost any company plan. Phone companies might have the next 2 or 3 generations of a phone mostly all planned out, yet they put off production until they know they've squeezed everything they can, both in profit and info, from the current gen. So riot puts out a gamemode, get the feed back and increase in player base, then they take it away so they can work on it while also funneling more people back into ranked.