By making Korg leave, it brings consequences in the current last main quest, and another that is being created (well, I think, then everything suggests that the sequel concerning what is happening around the city , that it will be a big very important decision concerning the possible future war between all the towns and villages, which risks being devastating for all, then we must not forget that Nordcrest is currently run by shapers = invisible demons who take the shapes they want).
And without the collaboration between Everet and Korg, all is lost in advance. - all towns and villages will be completely devastated by the red orcs.
Moreover, some paths (especially underground) will never appear on your map. - and consequently you never know certain strategic places that are linked to the main story.