Falling down and trying to catch your balance while moving in the direction of your initial momentum is not a football move. Had he changed direction in any way, it may have been different. In addition, I think if the defender had not gotten a hand on it, the call may have been different, but as he was trying to gain control of the ball after the defender touched it, it really brings in to question when true "control" of the catch took place, or if it was true control at all. Besides who is to say that they would have won either way. They would have given the ball back to the Packers with over 3:30+ left on the clock, plenty of time for 12 to move them into field goal range.
For arguments sake, here is the official NFL verbiage on the rule in question.
Here is what Rule 8, Section 1, Article 3, Item 1 reads: "If a player goes to the ground in the act of catching a pass (with or without contact by an opponent), he must maintain control of the ball throughout the process of contacting the ground, whether in the field of play or the end zone. If he loses control of the ball, and the ball touches the ground before he regains control, the pass is incomplete. If he regains control prior to the ball touching the ground, the pass is complete."
NFL vice president of officiating Dean Blandino
tweeted shortly after the game: "Bryant going to the ground. By rule he must hold onto it throughout entire process of contacting the ground. He didn't so it is incomplete."