Ranked Balancing

I was on a 7 game winstreak in solo/duo ranked. What ruined it was a yasuo player that did horrible and had no map awareness, did not help the team, etc. I was wondering why this player wasn't so great so I checked his opgg. What surprised me was the winrate. Why does he have a 9% winrate and he's still allowed to play in an elo that is above his. He has 1 win out of 11 games. I understand that he was plat last season but his mmr should not boost all the way to plat because he was plat last season, especially if the player is not doing well in ranked. It disappoints me that players with such a bad winrate and such low game experience is allowed to be in a ranked game where the ranks are higher than his.