User blog comment:Leyrann/Placement Matches - The toxic that creates the Elo Hell feeling/@comment-4929329-20121128034813

The critical flaw in your math is that your player has a 67% win chance. Realistically, that person should have a quasi 100% win chance vs. people with 1000 less Elo.

Rating based on KDA doesn't work as supports will have a hard time gaining Elo, it'll just turn matchmaking into an "all carries" game system. The reason it's only based on win/loss is because the point of the game is to WIN, and not to get the highest numbers.