Riot doesn't want a rank system based solely on player performance, but what about a system that utilizes both wins, losses and player performance?
Let's say you win a game, even if you played poorly you'll still gain LP or win your promo. But if you did the least work on your team, let's say you were 2/7/6, you would get less LP than the best player on your team, who was let's say 10/1/8. Obviously it wouldn't rely solely on KDA, but there's a lot of data in the game like warding score, damage to players, objectives, healing, damage taken, damage mitigated, etc..
If you lose a game you'd still always lose LP but if you still played well you would lose less. It makes sense to me that if you try really hard to win after someone on your team leaves, that you would loss less LP than the leaver.
Now I'm not advocating something crazy like 10x more or less LP. Just a few points, 3-4, based off of the individual performance for that game, independent of MMR.
Let's say you usually get 21 LP for a win and 17 LP for a loss. You lose a game but it's really close and you were 6/3/8 at the end. Well instead of losing 17 you only lost 15. Or maybe you win your next game and go 20/1/5, so instead of 21 LP you get 25.
Just food for thought. Haven't put too much thought into it.