I had a game as Karthus, my lane opponent was a leblanc. I was 8/3/4 by 15 minutes where they all decided to surrender. I would however like to point out that it was a mirror match at bot with luxes and jhins and well... Our Lux got first blood.
All I had to do at that point is... Press R every time they decided to fight so they win every time even if their jungler comes or the enemy got away with a sliver of health.
By the time they surrender the enemy jhin was 0/6/0 and the people who got kills on their team were leblanc and their lux.
Oh right, just to emphasize the how bad the enemy team was doing. We had a 10,000 gold lead and 4 towers and a cloud drake, they got no objectives.
In my opinion whatever team gets first blood and first tower win the game, with maybe a rift herald. Because the minute you get the lead you can pretty much do anything you want besides be an idiot. It probably doesn't help that ADC and support items are dirt cheap.