I'm not too sure why a lot of the player-base believe that winning or losing their placement games has a factor on where you are placed.
You could go 0-10 in your placement games and be placed in low plat.
It runs off of how you personally play, taking into account your CS at certain times, your neutral objective control and towers, Vision and ward coverage, how much profanity you use, how well you rotate, even down to your flat out KDA in your role.
This is why most people have an issue when they win all 10 of their placement games and they get Bronze.
Because going 22/0/0 with no towers, no objective control, 50 cs and placing zero wards in a team based game means you are out for yourself and have no business at a higher level where team-play and synergy is a must.