Yep.
Close, but not quite. You can rig a ban with the proper circumstances to prevent a secession. Assassinations, wounds in battle, in prison, away from the city, etc., would all let you get rid of a duke without allowing the to secede or change allegiance first.
Right... but it's still a fair compromise. Extra options could be nice, but we can work with the tools at hand good enough as it is.
