It's impossible because it's not possible to separate individual contributions to a team's victory in a manner that doesn't arbitrarily define what individual actions are most important to a team's victory - i.e, it's not possible to determine who on a team did 'the best' without having a strict definition of what, exactly, 'the best' is actually supposed to be, and to have that Riot would essentially have to pare the game down to a binary contest of specific measurable values, thereby enforcing a metagame that rigidly opposes the concept of strategy and more or less fundamentally ruining the game.
The healthiest possible thing to do for the solo queue ranked system would be to stop pretending like it's actually possible to accurately rank skill levels for solo players - this is especially obvious when you consider that, for the most part, having a solo skill ranking system in the game is more or less meaningless anyway?