Riot will not punish people for one AFK, although it happens a lot in SoloQueue, sometimes people actually have serious things that come up that need to be taken care of and take precedent over the game. Although it is very annoying to be on a team with someone who says they have to go sometimes it is just more important than the game. These players make it difficult to put the "You AFK'd a game and should be punished" stamp on anyone who leaves a game. Which is where this system would be abused, if you were in Promos or something and duo'ing with a friend and were clearly losing, you could easy just ask your friend to leave so you could have another chance at winning your promos.
The only system that I can think of for trying to help with the AFK problem would be to reduce the amount of LP lost for the team with a leaver or AFK'er and cause that player to lose the combined sum or the amount reduced. So if each player were going to lose 20 LP, they would instead lose only 13LP while the player who left would lose 48 LP (Original 20 + (7*4)). This way if the player has a real excuse and has to leave they will only be losing more LP, but it may deter some players from leaving and just trying to finish a Game they would AFK.
This may lead to a problem of Feeding rather than just leaving or AFK'ing which is actually more annoying to have on your team IMO. Just my take on the subject.
