Moving the capital costs a hefty sum of gold though, doesn't it?
Also, if the realm's capital is not under its control, it seems to still say that its capital is "Citysville". For example, when Wudenkin moved to Fronen, Bara'Khur's capital was Wudenkin for a while. If I guess correctly, after the bug that caused those who chose Realm A whose capital was held by Realm B to join Realm B, that was fixed so...people can't choose Realm A so long as its capital is not under its control? At least, that would be the case for immigrations, while duchy-specific ones would require no duchies and that would be a bigger problem anyway.