I don't think it's so much the capacity of the bridge as it is the design of the intersections and approaches on both sides which is in question.
But, I agree with you, I think aside from the height of rush hour, there shouldn't be too much problem.
I wonder what route they are advising truck traffic to use on the way over to Riverview? In many ways, the best route would be down Wheeler Blvd to the causeway traffic circle anyway, and then up West Main Street to Vaughan Harvey Blvd. Any other approach would entail too much street traffic.