like said, noone is playing in vacuum, especially not those on highest positions, and presented flexibility looks only good for now.
i cannot imagine why ruler would like to have too many duchies. too many dukes would mean inflation of ducal power and rank, and indirectly inflation of ruler as well (if he has to deal with too many subordinates, he is becoming kind of sergeant).
the same way, too few dukes in overly large realm would mean having too strong dukes who could eventually challenge ruler power.
the best thing is that, having no limitations, those in power will need to find themselves what is best of them, instead of being asleep in fixed game mechanics, and their searches should create in game fun.
what is also important, there is enough background for this, so medieval flavor will not be degraded.
