The glaring issue with placement matches and promo games
The problem with placements is the same exact problem with promotion games.
To find your true elo you're supposed to play X amount of games and win over time, so why in the world does a few promotion games or a handful of placement games affect your MMR and/or progression by so much? Even someone like Faker has what? 55% total win rate in solo queue?
Someones skill cannot be judged by placements especially in a game with so many random variables.
Yes, I know the argument of "well if you deserved such and such rank you could climb back to there". Yes, that is true, but it seems more like a gimmick to get people to keep people playing by grinding back up because there is no fair, nor logical reason for these arbitrary systems to be in place except to benefit Riot Games.