SoloQ MMR/Rank should be based off your Solo Performance
Why is your SoloQ rank based off your win or loss of a game? This is team dependant. This makes sense for Flex Q.
SoloQ should be based off your Solo Performance in the game. You know that Rank you get at the end of the game that shows your performance statistics in comparison to others playing that role/champ in your rank? This is what you should gain/lose MMR from.
Tell me why this isn't a good idea.