I'm not sure breaking Marienburg's monopoly will make it less wealthy. Tarrifs and tolls tend to stiffle trade and be counter productive, while a more competitive marketplace tend to encourge innovation and growth.
Sure, canals mean that Marienburg will now face some competition, but provided they adapt even a little bit, they hold huge competitive advantages : prime location, pro-business environnement, relative freedom and democracy, enormous accumulated capital, cosmopolitan population, longstanding protectorate of the trade hegemon, etc.
If anything I think Marienburg might be headed for a golden age paradoxaly.