You'd have to do a whole lot of pruning for power to pass to someone who doesn't particularly care for getting obscenely rich.
The idea would be that if Marienburg is losing asymmetrical warfare they wouldn't want to start something serious. If the merchant princes can be killed by shadows in their homes Marienburg could either go on the offensive and probably lose terribly or, hopefully, come to the negotiating table, maybe gain a few concessions but otherwise back down.