Loss Prevention for S Rank
It's a pretty simple idea, at least in my mind. Nothing feels worse to me than getting out of a solo queue game and seeing that "you or a teammate earned an S rank" message pop up after a loss.
Riot is basically saying to the player: "Hey! We know this loss wasn't your fault. You played better than X% of X Champion's players, but we're taking your LP anyway because your team decided to go for baron while the enemy was literally at your nexus.
I was actually demoted back down to Bronze 5 on a game where I played Karma support and received an S- champion mastery rating. It's no problem for me to climb back up on a day when I'm not tilting, but it would be wonderful--not only for my own mental satisfaction, but for the anti-tilt it would provide if earning an S-, S, or S+ netted you a ranked loss prevention.
