I see this causing an additional problem of teammates flaming their worst performing member to AFK so that the rest of the team can get lower LP losses.
Just have to weight out that negative with the current reality of AFKer's having the "power" to make their whole team lose full LP by afking.
You are describing what we call the "degenerate case". This comes up a ton in discussions about any system that will impact a large number of players. They will always be abused in some way by a very small percentage. We take this sort of thing very seriously and put a good deal of effort towards mitigating these cases.
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