The point is, the Queensland team has a very specific formula that has worked for them for 10 of 11 series so far. With Cameron Smith, arguably the greatest player of all-time at dummy half, the selectors have rarely felt the need to select a back-up dummy half and have often selected a utility to come on and play as a loose forward. The only exception was in Game 1 2008 when the Maroons selected PJ Marsh as the back-up and even then I believe Smith stayed on the field.
So it isn't a matter of McCullough vs Friend vs Granville vs Segeyaro but rather which player can wear the most amount of hats, who can gel with the rest of the team and who can threaten the Blues more. In recent years, the answer has been Michael Morgan who benefited from JT's presence in the side (just like Karmichael Hunt benefited from having Lockyer there) and will in all likelihood retain his place even though he hasn't set the world on fire at representative level.
When Smith does step down either next year or the year after, McCullough has as good a chance as any to replace him. It's going to be a tall order for anybody who comes in, but if Macca continues his run of form and can build on his efforts I'm sure he'll provide a decisive advantage over whoever the Blues select (Peats? Koroisau? Brailey? Liddle?...)