Win/Loss Streaks

On someone else's thread that was talking about the "conspiracy" of the MMR system being rigged, I'd gotten into a discussion about whether or not the conspiracies had any basis in reality. Lo and behold, a simple look at Riot Support's Matchmaking Guide page gives us the answers.

Climbing the ladder

While MMR is the number we use to determine your opponents, your personal progress is represented by the Leagues system. Here are a few familiar situations to illustrate the difference between your League and your MMR:

• You’re winning a lot, and the matchmaker is placing you against stronger opponents.

• A Silver player pushing for Gold should expect to see higher-tier players in their games. It means you’re winning more than you’re losing, and it means the matchmaker is starting to suspect that you’re worthy of higher placement. If you’re on a big win streak, you may even start to see opponents several divisions above you.
• Your opponent is losing a lot, and the matchmaker is placing them against lower-ranked opponents to find them an appropriate spot on the ladder.
• The team’s average MMR is the same, but one team has five roughly-equal teammates and the other team has a few very highly-ranked players and a few lower ones. This one is pretty rare outside of full-premade teams, but we make our best efforts to try and ensure that we still balance queue times into fair matches.

This explains the tendency to see long win streaks followed by long loss streaks. In its attempt to place you against what it believes to be more suitable opponents quickly, it's likely you may be placed against a higher-ranking player who's simply on a loss streak, not actually on your level. Which, lucky for them, will probably end their loss streak. But it will also hurt your attempt to climb.

It's important to note, though, that long streaks aren't all that common for someone genuinely ranking up (as opposed to a Diamond 1 smurf). But when they do happen, the way the system is designed actually does place you against opponents outside of your skill level, because it incorrectly assumes that's where you belong. The same as it's incorrectly assuming your skill level is where the opponent belongs. The system doesn't account for the fact that with a 50/50 shot, long streaks are still expected outcomes from random chance.

The only methods to remedy this tendency would have the side effect of increasing queue times as it searches for an opponent that is actually on par with you. It seems Riot is vehemently against increasing queue times though, even if it means putting players who will almost guaranteed lose the game for their allies autofilled into Support. And they tend to take a stance that if it's been "working" for this long, it doesn't need fixin'. So it likely won't be changed.