When you look at the arc, there is still tons to do.

Notice how the Wall situation is moving slowly so, eventually, they have to introduce Mance Ryder and his armies, including the minor Wun Weg Wun Dar Wun arc.

They have barely scratched the surface with Coldhands.

As other's have mentioned...Arya is soon about to become a favorite.

I think they have enough to pull from to keep it moving.