Even if someone has disconnected from the game for five minutes, that doesn't guarantee a loss. I've had games where a person disconnects from the game for 20 minutes, only for them to come back, apologize for having a cruddy connection/computer/whatever, and then we win. A five-minute disconnect says nothing about the outcome of the game.
That said, I don't know how to deal with people who decide to quit. I was pretty irritated when I noticed there was suddenly no difference between people who "quit" and people who "disconnect" (the former only popped up when someone legitimately left the game and the latter only popped up when someone did not hit the "Leave game" button, or something like that). It would be easier to tell whether or not a game will be irritating if we can see whether or not the person quit or merely disconnected.
And I'm all for the suggestion that these AFKers can be replaced with bots. I'd rather have an intermediate-level bot than an AFKer on the team. That, of course, would be difficult to implement.