So this placement matchmaking
Games 1-3: good teams, good hard fought games.
Game 4: "I only play mid, you forced me adc" said nothing in select. Leaves. Lose.
Game 5: "This is my first ranked game ever." -> all 4 teammates. Favorite line: "what do you mean trade? I wanna play mid". Massive feeding from all, lose.
Game 6: Guy plays ADC by clicking on the enemy and standing there AAing. Doesn't understand what he is doing wrong. Assumed younger sibling stealing account. Lose.
Game 7: "I'm carry from bot Morg" -> constantly engaging on the mid solo. "I'm support I'm supposed to die" after making the teamfights 4v5. Combined with feeding top and meh jungler, lose.
Game 8: Oh my god this is the worst mid I have ever seen, bot also getting super fed. Easy victory.
Game 9: Oh my god this is the worst mid I have ever seen, top also getting super fed. Horrible defeat.
Game 10: Enemy mid leaves. Easy win
Result: From Silver V season 3, with a 5-5 placement, I was put in BRONZE III. So a 50% win rate is worth a half a league worth of demotion. Yes, great and fair algorithm you have their RIOT.
This fits with my general experience with matchmaking, which is that if I start to win a lot in a row, the system responds NOT by making the opponents better, but by providing a streak of horrible players on my own team, and if I lose a few due to this, it reverses and makes the enemy team REALLY bad for several games. I do not find either setup particularly fun.