You do realize that the amount of people that AFK on your team is roughly the same amount of people who AFK on the other team? The only reason you don't think it works that way is because our brains are designed to remember 3 times as many negative experiences than positive ones. It's evolutionary. Something bad happens while hunting and you almost die, you remember so you can survive. It's scientifically proven that our brains remember more negative than positive.
Now, with that in mind, If you are like me, then you have won promotional games because of an AFK on the other team. You have won ranked games and gained LP because of AFK's on the other team. It happens roughly the same amount of times to everyone. Ranked is an environment where people are playing at their best to win. They are more stressed, more likely to get frustrated, and more likely to lash out and act irrationally. Do I think the League system needs to be changed? No, it works fine. If it didn't, Riot would have changed it. They changed the ELO system after 2 seasons because it was problematic. Dealing with AFK's is part of the game, ranked or otherwise, and therefore there should be no "well since you had an AFK and lost then you shouldn't be penalized". You will gain that LP back and gain more after that if you are good enough and deserve it.
It happens to me too. Yes, it is frustrating, but come on. It's really not that big of a deal. If you are meant to advance, you will advance regardless of AFK's or otherwise.