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There needs to be a system in play, now more than ever, to deal with a situation like this. A 4v5 is incredibly difficult to win (obviously), and it punishes random players who are paired up with said leaver/afker. It is far too common to witness a player threaten his/her team with "ok, I'm afk" when the team refuses to submit to the wishes of said player (ie, surrender, or I'm afk). While reporting attempts to mitigate damage, it is slow, and ineffective; leavers/afkers will always be present. It is akin to punishing a criminal after his/her illegal deed, without implementing any prevention strategies.
My suggestion is that Riot establishes a system that nullifies a game that is unbalanced in such a manner. This system would be similar to the situation that occurs when Riot is installing a patch; the game ends in a draw and no LP/MMR/IP is earned/lost.
To avoid a game ending in a draw from a person with a temporary problem, the system might activate only after, say, a 5-10 minute absence. The time period should be enough so that the player has a chance to correct his/her situation, but not so much that even if he/she comes back, they are too far behind.
Abuse would be a clear problem for such a system. Therefore, I propose that any players found to be absent in game should be punished with a 24 hour ban. Perhaps the punishment would increase if the offense is repeated. This should be severe enough to deter anyone from leaving a game just to avoid a loss, but not too severe to avoid wrongly punishing a player who lost connection due to circumstances out of control (perhaps a thunderstorm, or power outage).
Implementing such a system would decrease the number of losses/wins due to circumstances outside of gameplay. As you no doubt have guessed, this post is a result of my experience in a game with too many afkers. It is simply a horrible feeling to have lost a game because of an afker. My specific situation is an afker in one of my Bo3 matches to promote (twice actually. I lost one of my first Bo3 due to an afk, and the second due to bad play on my part. I am now about to attempt to win 2 games in a row. But losing the first in Bo3 due to an afk is very demoralizing).
Riot, I implore you. Take care of this situation. It is a plague in the community of League of Legends. This must be fixed.