Okay, let me put this "problem" of unintentional feeding in a different perspective.
Unintentional feeding is something that cannot be accomplished by one person. It takes two. One person to die, and another person on the other team make the kill.
What if Riot implemented a system to punish both people who were responsible for the unintentional feeding? The player who had the really bad KDA and the player who kept killing him. This would put a REALLY fast stop to unintentional feeding, don't you think? As soon as you realize that you have a lead on someone, you would stop fighting them entirely, even when they can easily be killed, even when they clumsily engage on you. Failure to comply would mean that your account gets restricted or banned, maybe even permabanned. There would definitely not be any more unintentional feeding.
That's ridiculous.
It's ridiculous but it would absolutely work. I'm sure you don't think it's fair, but why not? You're winning the game, you have some items, and if you keep making the decision to position properly, use skills effectively, buy the appropriate items, and focus on the correct enemy, you're going to ruin the game for a bunch of people who got stuck on the same team as that feeder. But you're not doing anything wrong, are you? You can't help it if he's bad. The game is about making good decisions in order to get your team closer to winning; you can't help it if your decisions are right and his decisions are wrong.
And he can't help that either.
The failing here is not the bad player's failure. How is he bad? Just because he's worse than someone else, that makes him bad? Maybe the bad player in this example is in Silver, and the good player is Gold. Maybe the bad player is low Diamond and the good player is high Diamond. Maybe the bad player is xPeke and the good player is Nagne. It's relative. And saying "you should be punished when you're pitted against someone who is better than you and you do worse than them" makes EXACTLY as much sense as saying "you should be punished when you're pitted against someone who is worse than you and you do better than them."
I understand that you're looking at this with the goal of making the ranked experience better. And in your view, when someone on your team does really poorly, that makes ranked suck for you. But there is no possible way of making this go away. One of the two teams is going to lose, and half the time it's going to be your team. One person on your team is going to have the worst performance, and four out of five times it's not going to be you. If that seems like something to get upset about, if it seems unbearable that you will frequently lose games because of someone else's unintentionally bad performance, then you may have to re-examine the reason that you play a team PvP game. Because people will always lose. People will always feed. It's relative.