Well not sure if I would go that far but I guarantee Stewart attacking in force at Meade's rear would have dramatically changed the outcome of the third day of fighting.
It might have turned the tide for the South or maybe extended the confrontation another day or two. But the devastation to the Southern forces on that third day forced Lee to disengage. Stewart could have changed that dramatically!
I think Stuart’s role was not so much to cooperate with Pickett in the attack as to exploit the confusion created by Pickett’s successful attack.
No success by Pickett and no resulting confusion would have meant Stuart attacking formed infantry with cavalry, which in this war never worked out very well.