Despite the general vernacular understanding of the rule, it is not necessary for a receiver to maintain control of the ball as he goes to the ground unless previous control was never established, control being defined by Rule 8, Section 1, Article 3 of the official 2013 NFL rule book as follows:
"A catch is complete... if a player, who is inbounds:A and B were clearly met. C is the item at issue, and by its nature is almost unavoidably subjective, but it needs to be kept in mind that the parameters for overturning a call made on the field are, according to Rule 15, Section 9, Article 3, as follows:
a) secures control of the ball in his hands or arms prior to the ball touching the ground; and
b) touches the ground inbounds with both feet or with any part of his body other than his hands; and
c) maintains control of the ball long enough, after (a) and (b) have been fulfilled, to enable him to perform any act common to the game (i.e., maintaining control long enough to pitch it, pass it, advance with it, or avoid or ward off an opponent, etc.)."
"A decision will be reversed only when the referee has indisputable visual evidence available to him that warrants the change."If it was plausible that Bryant was stretching for the end zone after both feet made contact with the ground, which would constitute an "act common to the game", it shouldn't be overturned.
Is there indisputable evidence that Bryant would have clearly lost his footing even if he were not stretching for the end zone? Quite possibly--heck, probably--he would have, but it's conceivable that he might not have, and that bare minimum possibility is the standard for the ruling on the field to stand.
Had the pass initially been ruled incomplete, it'd be an open-and-shut case, but the ruling on the field was a completed pass and thus should have been left to stand.