How would smurfs even get to bronze... if they were really smurfs they'd have been placed at least high silver and would never have seen bronze?
Lots of players probably just claim to be smurfs to get their preferred role. Or you just assume they're smurfs because they got fed - newsflash, even a garbage player will dominate the game when fed hard.
I don't disagree that it would be impossible to climb out of bronze - anyone who doesn't play super aggressive to get fed fast and carry hard (but instead plays a farm-oriented, passive game and just gets a couple kills) will be at the mercy of whichever lane feeds and has to hope it goes their way (and at a 50-50 record you'll never climb).