I agree completely(except the part that you should have been able to carry a game with 15 or 30 kills, most champs can't win a game by themselves, no matter how fed they are:) ). Far too many players think KDA is all that matters. Despite how obvious it is that other things are also important, a good portion of the players seem to to totally convinced it is the end-all in this game. So much so that they'll start ignoring teammates, often going out of their way to get more kills at any cost, and encouraging people to ignore supports and any other players that don't have a "good" ratio.
Anyway, it would be good to put out a video at least explaining how a guy with 22 kills still dies fast when he runs around by himself, but I'm afraid most players won't even watch it(the Teamwork OP video was similar, and despite it being correct, and one of the most common problems on here, it seems like not too many players really took that advice).
The main problems with the KDA are objectives and how people become so obsessed with score that they don't care about the actual game and the required teamwork. I mean if your Riven has 18 kills and the entire rest of the team has 7 combined, its time for her to stop running around insta-clearing every jungle camp and specifically holding attacks until they can last-hit champions. Then the biggest issue would be pushing a lane. You'd think such a simple and important task to the game would not be so hard to accomplish, but it is. You try, and try, and finally get the team together in a lane, kill maybe 2 minion waves, and more often than not, everyone ends up scattering off to go do random stuff and starts getting picked off. You explain that we just lost a bunch of turrets because we spread out, get the team in one lane, and the instant someone walks off to ward a bush, the team scatters again. Yikes!
Here's hoping something happens that changes this, as it's getting really, really old.