I disagree with your idea. I don't think that a person should be punished for leaving a game because sometimes the person simply has no control over it. Would you like to lose thirty or forty lp because your internet went out on you, or the client bugged, or you computer froze or your power went out or the world exploded? Of course you wouldn't! However, while I've become more and more convinced that the ranking system does overall work (being that if you truly deserve to go up in rank, then you will ultimately win more games than you lose, even if sometimes it doesn't feel like that), I do think that an afk should be counted as a loss forgiven, especially if you're in a placement or promotional.
As far as other people leaving, there really shouldn't be any excuse for you to leave if someone else does. Indeed, there is no telling what might happen if you stay the course. I just had a match earlier today where my adc-graves was afk for the first eleven minutes. But we still won because he came back, and not only that but the enemy tf left as well.