Hey, people are cheap. As long as the bridge isn't in danger of collapsing, which if it was it would be closed immediately, people can use it. And if more people use it, then it'll jam up the bridge, and people can decide if it's worth it to skip the dollar toll in exchange for a slower ride.
In the very immediate Trenton area, the best crossing over the Delaware is the US 1 Bridge. It costs $1. But people have the option, and often use it, to exit and take the free, single lane, 25 mph bridge, going thru a few traffic lights in Morrisville, PA.