It's common for dukes to hold their positions for real life years - decades even - and few, if any, of them would be banned, even if it were possible. Friendships go back; running a city requires little attention, no skill; dukeship is the penultimate position, to be kept at all costs. The only way to promote duchy turnover would be to increase the likelihood of mortality. Say, allow infiltrators to really assassinate - no more rubber daggers. But that isn't happening, people don't want to lose, this ain't the capitalism.
