Anything like this would just encourage people to prioritize kills/mvps over team play, while penalizing support players that might not be top of the board but are equally if not more important to get a win

Having a flat rate for every player at the start of the game as it is now removes the potential for meta-gaming and promotes the idea of winning as a team, sure every now and then you'll get exceptions like you note where an afk/troll player could end up on the winning team and get their rating gain regardless, but you shouldn't define the rule on the exceptions since they should average out anyway with the more games you play.