It's not a theory, it's what happened, both have had teams and coaches try to turn them into 7s and both did not work. If now that they're more experienced they can go alright at halfback is a question worth asking, but as it stands they were both nothing more than unfulfilled potential when they were playing at 7, and games took off once they were put in the 6.
Well I'm not sure what goes on behind the scenes, maybe players put unnecessary pressure on themselves because the number has changed. Anyway, based on how they play for their clubs I don't see why NSW can't make it work. I don't expect them to play like Johns or Thurston but I think it's reasonable enough to ask Carney to run the left side attack like Maloney does for his club/state and Mullen to run the right side attack like Pearce does for his club/state, or vice versa. "five eighths" don't really play the traditional role anymore, IMO they can't play like a halfback then they're useless.
Anyway Pearce has only been NSW halfback for as long as he has because his only real competition are "five eighths" that are never considered against him, so he has virtually been unchallenged.