nice false dichotomy. Got some logical arguments to make?
You seem to presume no other routes existed or can exist. Also, how is it beneficial for the toll setters to have tolls that throttle a route to extinction? Toll roads today have low prices because if they didn't, people wouldn't use that route. If everyone had a chunk of road and charged for use of it, people would avoid expensive toll routes to save money, which would force that route to drop its prices to make any revenue.
Competition forces prices down, and someone would drive a road through to bypass the river route if it became too pricey. Before it got to the level you describe, someone would sell their barges and start negotiations with people to drive a railway between the two towns. Alternatives to the dystopia you describe exist, and people would seek them out. Just because you can't think of a way doesn't mean no one can.
Your example of what voluntaryism leads to is so simplistic it becomes a complete strawman of that system. Got logic? Because this ain't it.