In my early years, we had a few mechanical bridges that crossed local waterways with commercial traffic. Two were draw bridges, and four were swing bridges. They served their purpose, and were marvels for their time, but being mechanical, they had problems.
One drawbridge, when it was raised for what would be the last time, was left in that position. Another drawbridge, had an electrical fire in the raised position, which left it in that position until demolition. Both left some important areas without access, except through long trips that would take over an hour. The solution was temporary ferry service, but the ferries could only handle pedestrians. Those that were affected left cars on either one, or both sides, for carpooling. Inconvenient is a mild term to describe the situation.
The swing bridges were less of a problem, but cantankerous machines. Problems would arise, parts might not be available for the old bridges, and the costs to operate prohibitive. The solution for two of the bridges was replacement with spans higher than any boat traffic. The solutions have been there for years, and those affected never affected by mechanical problems since.
Two of the swing bridges remain. Both have had expensive repairs, are still cantankerous, and loved by those historically minded folks that wouldn't be so anal about the bridges if they had to pay for the costs associated to keep them in service. How long this lasts is to be seen, but as time goes by, and those without any emotional ties to the bridges are gone, they'll be replaced, and the extra costs for keeping the old bridges absorbed by the taxpayers that probably don't know they even exist.