The ball was catchable but anything but a routine catch. The pattern Welker ran was to receive the ball in the inside, not on the outside. So he had to almost do a 180 and catch a laser. If you watched the replay they showed during the game, they did show a closeup of his footwork during the play. Because he twisted his feet around, he could not plant and had no leverage to jump up and out towards the ball; he was just falling backwards.
If that catch was a touchdown or inside the 10 maybe you could say it's the game. But anything could happen after that; a sack and fumble, a pick, a missed FG; etc.