Riot doesn't force a 50% winrate.
I had 9 wins and 25 losses going into a ranked queue of Bronze III with a hot 7 game losing streak, and got pitted against an enemy op lane that was three brackets above, in Silver. The enemy bottom laner was also Silver, while the rest of both teams where in the appropriate placement bracket.
At the end of this match I am predictably now at 9 wins and 26 losses, and the mythical 50% seems to be nowhere in sight.
I'm never going to win a match in this game again am I? Let alone get an S for a mastery chest. I really sincerely wish that supposed forced 50% would exist, because I could use about 16 wins.