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Repeated AFKs are already punished by LeaverBuster and losing LPs, even if they reconnect before the end of the game. You cant double/half the losses/gains because it would mess up the ranked ladder and the correlation with MMR. Exception is loss prevented activated for everyone during a server issue. And probably some messed up ways to abuse with smurfs and win trading.
That being said, AFKs dont damage the ranking but they damage the fun:
Regardless of their %, AFKs are irrelevant to your ranking for a simple reason: they can ALSO be in the opposing team and allow easier wins. Use your memory and remember that you lost some games BECAUSE of AFKs and you won some game THANKS to AFKs. Over a Season of hundreds of ranked, their presence evens out = no impact on your ranking. Simply our brain is wired to stronger remember bad experiences.
That being said, its frustrating of course and should be better identified and punished, hence the announcement from last week. Foutain AFKing or running around should be targeted indeed.
https://euw.leagueoflegends.com/en-gb/news/dev/dev-ranked-and-matchmaking-in-2020/
It doesn't matter if i've had afks on the other team before or not, and one could also argue it's not a perfect 1:1 ratio, so it's not a good excuse.
They absolutely mess up the ranking system, it stops the progression that the game is supposed to have. I lost 4 different promotion series last season to AFKers on my team, tell me how that hasnt impacted my rank? If someone afks, it's not longer who needs to gain lp gaining lp, now its just the side that has 5 people so i dont understand the one dimensional thinking behind your argument. People can rank up that are NOT supposed to, and people that ARE supposed to rank up can be held back by AFKs. How is this not an issue to fix? It's clearly not working.