While having an AFK sucks, this would Ben highly abusable.
Situation 1: You’re in a premade and you are clearly going to close. Either you or your premade leaves, preventing the other from losing any LP. Sure, maybe you or your premade loses LP, but at the end of the day one of you can just hop on a smurf and prevent the other from losing LP.
Situation 2: Let’s say riot inputs a situation where if your premade leaves, you still lose LP, but your other teammates don’t. This would hurt duo queuing, as if your premade or you loses internet connection or RIOT servers bug out, the other person would lose LP just because you or your duo had connection problems.
Situation 3: Same situation as above. However, you or your duo don’t want to make the other person lose LP. This will create toxicity, as people will just bully one another to leave in order to prevent the rest of the team from losing LP.
As you can see, it would be extremely hard to input a system without it being abused.
Cheers!