I don't think you appreciate the magnitude of the ongoing maintenance costs for a bridge. Regular cleaning and repainting of the metal structure to prevent deterioration, regular X-ray examination of the steel to check for impending structural failure, repaving more frequently and with more expensive methods/materials than land roads due to more extreme temperature swings, etc.
Didn’t they have those costs while they paid off the bridge? They were able to meet those costs and maintain the bridge.
I’ll bet you would be astounded to know how much money the Golden Gate Bridge takes in each year.