PSA: stop whining about "forced 50%"
This "forced 50%" meme comes from a complete inability to apply basic logic. This isn't some evil Riot method to make ranking harder. This isn't some sinister thing Riot does to hold you down. You just don't get how statistics or rating systems work.
The purpose of a rating system is to estimate the probable outcomes of a match based on the two competitor's past performance. Then, based on the actual outcome, it adjusts the rating. Here's an excerpt from the Wikipedia article on the elo system (which leagues MMR is based on but not a perfect replica)
"The difference in the ratings between two players serves as a predictor of the outcome of a match. Two players with equal ratings who play against each other are expected to score an equal number of wins. A player whose rating is 100 points greater than their opponent's is expected to score 64%; if the difference is 200 points, then the expected score for the stronger player is 76%"
So why does Riot admit that the matchmaker looks for a 50% winrate?
Because the matchmaker by nature has to aim for some winrate
The matchmaker can't just match in a vaccuum. I mean, it could be random but I don't think anyone wants that. Point is, for it to actually be a performance based matchmaking system, it has to have a target winrate for each match.
This "target percentage" isn't unique to competitive games. For example, the writers of standardized tests like the SAT and ACT have a "target average score" they aim for - they try to design the test so that, say, a 70% score is average. For any system that attempts to mathematically evaluate skill or competence, you have to choose what the algorithm decides is average.
There is a major difference between League and the SAT (besides that studying one is much more likely to get you a job than the other). In the SAT, if you get a good score, someone else doesn't get a bad score as a result. You're only competing against yourself and the questions. In league, every game has 5 winners and 5 losers. If you have the same 10 players play any number of games, even if they randomly change teams, the overall average of all their winrates will be 50%. For every winner, there is a loser.
This means unlike the SAT, Riot can't set an arbitrary percentage of whatever they want. Average performance has to be 50%. The average of all summomer winrates in the game right now is 50%, beacause again, for every winner there is a loser.
The matchmaker uses the ratings of players to attempt to create games where each team has a 50% chance of winning. In other words, it tries to make fair games. For the matchmaker to attempt anything else is either mathematically unsustainable. It's not possible for the combined winrates of all league players to be anything but 50%.
I can hear them already "B-b-b-b-b-but whenever I go on a winstreak I get the retards on my team Riot is rigging the game so I can't rank up!" No. That's just confirmation bias. The reason you eventually lose a winstreak is because the game is putting you in more difficult matches. I will admit the use of average ratings of the team contributes to this flaw, because your rating is climbing and therefore the average of both teams can show an even match while individual positions are horribly mismatched. I don't know how it factors standard deviation of team member's ranks into its algorithm. But that will affect you and the enemy team equally. What's more likely is that you're fighting harder opponents and not willing to admit they're better, so you blame your team. There's also the fact that any winstreak is a statistical anomaly, and it will be corrected in time.
"B-b-b-but how do I climb if I'm forced to have a 50% winrate" If you have a 50% winrate over hundreds of games, that means you are at the average performance for your rank. If you want to climb, you have to be better than average. This means the matchmaker will boost your rating until your performance hits 50%.
tl;dr: 50% is an average performance. The matchmaker tries to find what rank you have a 50% winrate at. If you get above 50%, it puts you with higher rank people until you reach 50%, and thus an average performance for that rank. If you want to climb, have an above average performance
