$6.4B and counting, plus a quarter million vehicles per day; there is no possible alternate way to move that many people. It had to be replaced while the old one was still used.
Since drivers pay the $6 toll only one direction, it will take $6.4B / (125,000 vehicles per day * $6 per car * 365.25 days per year) = 24 years to pay off the bridge with tolls alone.