You know, one thing I dislike about this series is how everyone, including the girl herself, from time to time says something like "How could we tell a 16-year-old girl to govern the country?" or "What would I even do after killing Soo-won?", or even "Of course I can't seat myself on the throne."
Soo-won is just 18, you know that? Louis XIII "seated himself on the throne" when he was only 16 (Yona's age, incidentally), and that was late, he'd been of age since he had become 13.
Why does everyone rush to the conclusion that just because she's young, she'd be unable to rule the country as well, to say nothing of better, than Soo-won? That might have been true before she started living the rough life, but by now she's had much more experience of this world than Soo-won, whose only specialties seem to be court intrigues and warmongering.
Compare that to Arslan. I haven't read the original, but Arakawa's version is good enough. He's 14 and everyone around him has no qualms about his becoming the next Shah if he manages to pacify the country. He's inexperienced, but everyone agrees that a good ruler needs the best advisors (which Yona is sure to have) and wisdom to make decisions based on rational, sound judgement (which she has heaps of).
I know the author needs a reason to keep her away from Kuuto. She probably wants to make it so Yona decides to seize the throne out of necessity. She probably wants to first turn Soo-won into a villain (as if he weren't from the beginning) before she has Yona take that step. I know this is all a huge plot device to stretch the plot until all pieces are in place.
But she needn't have put these lame excuses on everyone's mouths. When confronted by Kyo-ga on revenge, Yona could have just said, "As long Soo-won doesn't do me or the country harm, I'm content with my current life" (which she kinda did), without voicing such a poor opinion of herself. Soo-won had no more administrative experience than she when he usurped the throne.