I had a plat 5 jungler come in telling me that you can always carry a bronze game. He was like 28/3/2 and we had a total of like 35 kills. 2 of those were mine as support Nami. That left 5 kills for the whole team to have gotten outside of us two. We still lost. I remember him remarking, "You guys are making this impossible to win."
Last night we had a great team. I was 5/3/33 Taric, our Yi was at 2x/5/x and Veigar was eating people alive if they got cc'd. Our adc picked graves which made me want to dodge at first, then he got countered by Cait so I told him screw it, I won't dodge, but let the wave push into our tower so Yi can gank easily. We won lane doing that despite the cait pick into graves. He ended up 5/18/8 by the end of the game because he REFUSED to back, listen to ping and constantly got caught out 1v4 without the team or vision. I kept telling people to get farsight wards. No one did. Ever. I no longer run sweeper. I get the farsight so I can give someone vision when they are being stupid. On the lucky chance I actually spot an enemy they might just run for it while they still can.
It doesn't matter if you've got a smurf or 4 great players, if you're dragging around dead weight (especially into late game) then you're basically fucked. You might win, but odds are completely against you. Even when you win it's still so aggravating that you can barely enjoy it. Some players really DON'T want to win.