I made a post earlier today about this. Simple solution riot could put in.
If someone AFK's for more than 5 minutes, the loss is prevented for everyone other than the AFKer, who suffers an LP loss even if the 4 man team can pull off a win.
If someone AFK's while duo queued, they both suffer the LP loss. This prevents manipulation by teams of people who would have someone go AFK in order for the rest of them to dodge a lost game. Otherwise four man teams could have one guy jump on the grenade to save the other three from LP losses. If the rest of them stay and pull off a 4v5 win, they still dont get any LP, but they don't lose any.
Simple solution, why Riot doesn't do it (it's not that revolutionary of an idea), is beyond me.