The real question is why anyone would push their property claim to the absurd state where they are destroying their own income, as in the scenario. There are privately owned toll roads and canals that exist right now. Why don't they keep increasing their fees at a rate that asymptotically approaches infinity? Because no one would pay them. The simple fact is that producers are not free to set any price they wish in the market. They are constrained by the willingness of consumers to pay their fees. If it were not so, the price of everything would be infinite.

The OP posits that there is a sort of reverse cartel defection effect at work here, where all the members of the River Transit Guild are induced to defect against a decrease in prices necessary to save their business, but in fact it would be the opposite. Raising the prices at your gatehouse while everyone else lowers theirs doesn't mean that you will make more money, it means that more people will avoid going through your gatehouse.