It's a nice idea, but it's a pipe dream. In an ideal world, your promotion or demotion through the ranks would be based on YOUR performance in a game, rather than whether you get dragged down by ten-thumbed cretins who literally don't even know how to play, or carried by diamond smurfs who descend from their ivory towers to stomp bronzies with Riven. Being assessed on your performance and your performance alone is the only way that would be truly fair.
However, it's impossible. Champion Mastery is a very loose, casual system which can never amount to anything more than cosmetics because it's too inflexible and too open to abuse. A player who focuses exclusively on actions that boost the Mastery score for their role, even while he completely fails to support his team, should not be rewarded for putting his S+ rank ahead of victory. Plus some champions have multiple roles, which means their score may not necessarily reflect whether or not they actually "did well" in the role they were having to fill (you know Nunu is classified as a Support, rather than a Tank/Bruiser?). So basically League isn't fair and all you can do is deal with it.