The Conqueror had only managed to make the North kneel because he had three dragons and Torrhen had been unwilling to see his lands and keeps burning. Myrcella did not doubt that Jon Stark would not be kneeling to anyone, ever. If he desired, he could probably conquer the south with his dragons. And he was a tried and tested battle commander and a warrior. With Shireen Baratheon for a wife, he would not meet much resistance.