Passengers don't need a "3rd" (second actually) runway. Aircraft movements might. But aircraft movements have yet to get back up to the level they were at 14 years ago. Demonstrated need for a new runway is still a long way off. And in the intervening period there will be (Trump notwithstanding) greater pressure on aviation emissions reduction and - I would predict - a change in the attitude that a new EDI runway is easy because it "only" involves concreting over farmland.