Have you seen this sign? I think it will not hinder the feeding of water fowl. If the parks really want to do right by the ducks and the visitors (and themselves) then what they ought to be doing is selling small quantities of proper duck food through onsite vendors and vending machines. Signage should focus on the importance of proper diet for the animals and the manner in which funds are used to help maintain public space and protect local wildlife.
No one benefits from the current system. Meanwhile the pastime of feeding ducks remains one of the great pleasures of the park experience, and isn’t likely to change or diminish with time.