Ranked losses

So today I played a ranked match and all was going well.. it was pretty evenly match until at exactly 20 minutes in... our jinx quits now making it a 4v5. we had hopes that since it was ranked the jinx would come back... maybe they just DC'd or are having connection issues though they never said anything about lagging. After 10 minuets or so of not even trying to reconnect we decided that the jinx most likely left on purpose and had no intention of coming back. probably being one of those people who struggle with time management "sure i can start a match i have about 30 minutes of free time before i have to go". Well.... the match lasted over an hour. we won almost every team fight. the problem was only one of us would survive to push back out 3 lanes. so by the time the one survivor got the lanes under control the enemy team was back up and we couldn't make any headway. so in the end after spending 40 minutes 4v5 winning almost every team fight taking all but 2 drags and we had both barons they caught one of us in the jungle and that was it. 3v5 for the last team fight and we couldn't do anything about it. Now what i want to know is why the hell did i still lose full lp. The game know's when someone leaves for an extended period of time. that's why it used to say "leave" in your match history if someone left a game and it didn't end within a certain amount of time.... our team did not suffer a loss do to being out played or out smarted or making to many mistakes. we had more kills more gold more wards drags and barons than the other team. so why is it when your team is... crippled in such a manner and you have no control over it are you punished like you got out classed and lost fairly. wouldn't it make more sense when the game knows someone actually left to cut the amount of lp that team is now going to lose in half. it didn't matter what we did the likely hood of us winning 4v5 when we didn't get a huge lead at the start is almost non existent. Riot should just start queuing people with a random chance of only having 4 people on their team just to see what the likely hood of them winning is. but that wouldn't make any sense. so why do i still lose the same amount of lp as if it was a full team. to me.. that makes just as much sense.