I agree that there should be something to combat this, but think of it from another point of view.
You join a ranked game as a solo person, and all of your teammates are solo. One is afk from the start, and you end up losing. If you lost no Elo, either the enemy team will gain none, or gain elo. If they gain none, they are not happy with the system. If they gain some, then you now introduced Elo inflation to the system, which over time is detrimental. This is why Riot is reluctant to push "the button."
Ahh but I have a brilliant idea: have the afker lose elo equal to 5 times the normal amount of elo lost, such that they lost 50, the other 4 lose none, and the enemy gains 10 each (obviously can be tweaked depending on the matchup ie 60 and 12). But now you open it up for trolling. Trolls can join a ranked game, and afk. They can now drop their Elo easier and faster than ever. Once in low elo they will make other games unbalanced due to their 'true' rank and their current rank. This system will not work either.
Also, as long as your are not the AFKer, nor is someone in your premade, you are more likely to encounter an AFKer on the enemy team than on your team.
I believe Riot is better off trying to make a system to punish AFKers better, which will make afking less prevalent.