I'm fairly sure there is a warning when a Duke is about to go inactive. As soon as that happens, any sensible ruler would start taking auditions for a successor, so one can be appointed ASAP if the Duke does vanish. A city that goes without a Duke for several days, only to see the one noble who was incapable of holding it in the first place reappointed, should at the very least elicit some furious response from the general nobility.
Also, it's just cheap from an OOC point of view.
