The reason why /mute all exists
Is because Riot won't let people backlash at dickheads in the game.
Literally just got out of a game to hear the people I muted flaming the living fuck out of me. Muting doesn't carry over into the lobby so while looking into graphs I get to listen to Mundo and Darius tell me how much of a fucking scumbag I am. I didn't necessarily perform poorly either, it just so happened that I was playing Pyke and the enemy team had 3 Zhonyas.
Riot shouldn't make people feel defenseless like this.