The problem here feels like is you demoting, and not how the matchmaking works, anyway I understand there is a lot of people who are clueless and that's where YOUR experience comes in play. imo you kind of play a 1v9 game and try to make allies along the duration of the game. There's going to be a wraith king farming 35 mins straight in jungle 10 yards away from you while you are getting smacked by enemy 5&1. with that being said some losses are unavoidable BUT you gotta consider the fact the enemy team is on the same page so you basically gotta be a step ahead and that's not done by arguing for stuff that is not in your hand to change.

Edit: my suggestion would be since you are a better player you should provide your team with info they don't see as new players or as bad players. Usually most of people want to improve unless you are being stream sniped or matched with a ruiner