Video should only be used to adjudicate in goal.
Why should the lead up play for a try be given more scrutiny than the play or plays that precede it?
As an example, If a team makes a line break from a forward pass, that goes unnoticed. runs fifty meters, is tackled, scores simply off the next play as the defence fails to set. It is still the forward pass that creates the try, yet that will never be and should never be adjudicated by the video ref.
The only difference in a play where a try is scored is the act of actually scoring the try. And as that can happen in only one section of the field, it's the only logical place to draw a line whilst retaining consistency in the way in which the game is adjudicated.