I don't think the 2nd day was truly decisive.
They still could have run around to the right of the Union position on July 3rd. Sure it would have been a very audacious plan, but Lee had lived on that reputation up until this battle.
The 3rd day was decisive. The 2nd day set the Union in the direction of victory.
Actually I think day 1 was the decisive day. When Ewell didn't take Cemetery Hill.