As with most arguments about the laws it becomes a question of fact.
The doubters, I think, do not accept as a question of fact that Potgeiter's contact was what stopped the try from being scored. That contact was substantially ineffectual is the view taken by doubters. No amount of referring to the laws as rules and looking at them will change the ways in which each school sees the facts.
So correct me if I have this wrong, but if the contact, which did occur thus an established fact, irrespective if it was ineffectual or not, was foul play; and foul play of any sort while a player is in the act of attempting to score a try invites the question of if the act of foul play could have have influenced the try not being scored, and in considering any decision the benefit of the doubt is with the attacking team?
So then when considering the available facts, it would be logical to consider that the player, as observed by Joubert at the time, was millimetres from the line (fact), and removing the element of foul play being the contact from Potgeiter's tackle, being to the head and front of the player, which it would be fair to judge that removing the contact element from the head or front of the player changes the dynamics of the tackle (fact as the contact is the foul play), which presents a case of based on the available facts at the time, and the benefit of doubt gong to the attacking team, it is most likely that a try would have been scored.
In establishing the above it has been considered as fact that player has committed an act of foul play. Irrespective of where, how or when the act took place, it is punishable exclusive to the circumstances of the play at the time.