If matchmaking was fair it would ...
Account for stats when pairing players together.
Many people don't like the idea of giving players more/less LP based on their performance every game, cause that could be abused etc. So having above average/good stats don't mean anything if you're not winning games.
Well, at least take those stats and use them to pair people together in a manner that could create a way better matchmaking experience.
There are sites that show up important stats about a player like : Aggression on Lane / Split-Pushing Tendency / KDA / GPM / Objective Participation / Vision Score and many many more. I don't think Riot doesn't have access to those stats. I also have a slight suspicion that they already use them but not in a pleasant manner.
Why not use those stats and create an algorithm that pairs players around the same elo that have low deaths per game together for example, or players that have high kill participation together, or players that have high objective participation together ?
I would definitely prefer having teammates with the same average of deaths per game as me even if they would be lower elo , than having higher elo players in my team that are careless and die many times each game.
Of course, a system like this wouldn't be able to pair together 100% teams of players with certain stats, but it should have a tendency to pair up players with similar play styles.