For the region it is 99% of the traffic, nobody will drive for half a day to avoid the bridge. But when the alternative opens (and it is not a toll bridge) you can expect traffic on the privately owned bridge to drop like a stone and likely it will eventually become public property.
They are if they are the result of an artificial monopoly and there is an alternative on the drawing board with better rates. In general infrastructure should be free to use, toll is friction on commerce and development.
Where are you taking this analogy?