In my view, it shouldn't be based solely on w/l. I'll use CSGO as an example, it's an extremely team based game yet it uses a hybrid system. Whether you rank up or down is still based on w/l, however they take other things into consideration. MVP medals, scoreboard placement, kills, and etc. So lets say you have a really good game, yet you still lose, you won't lose as many rank points. Same goes for the opposite, lets say you played extremely poorly yet your team still carries you and you win. You'll gain points but not that many.
I have no problems with w/l being the deciding factor, however I believe other factors should also be taken into consideration.