What you playind and which lane? If you are mid, your team has a carry or more, you down the tower after 9min and then the laning phase is over, then it's your fault you loose the game.
I play almost always with Sivir, mostly in mid or solo lane. And i seriously try to NOT take the tower down, because i have seen things go bad after i do that. (feeding on both ghosts camps and the wolves while defending the own tower is ridiculously easy with Sivir btw...)
About the topic, if you see that the matchmaking is broken, please try to be constructive. Like what riot could do to identify bad teams.
If it was a fact that a team needs a tank to win, riot could calculate the chance for people to choose a tank based on their last 20 matches. And then try to even this chance out for both teams.