Such is the show's blessing; such is its curse. It would be pleasantly surprising to see Gillen and Darvill stay on, as they make a great team with Smith (and Kingston). However, it looks like the current season will have a trajectory that takes them out of the show one way or another. Season 7 will no doubt have a different dynamic.That's another reason I wish the actors stayed on the show longer. They only have 13 episodes, and there are usually 2 or 3 two-parters in there. So really, each companion only gets a 9 or 10 adventures and then is put to pasture. Lame.
I just wish there weren't so many two-parters. If you only have 13 episodes, you should try to utilize them for as many stories as possible.