For reference, construction of the new, 3-mile long Tappan Zee bridge cost $4 billion and took about 4 years to complete.
They could take their time with the new bridge because the old one was still in place.
In this case, there is only one bridge and it's broken.
For more reference, the privately owned Gordie Howe bridge between. Detroit and Windsor, ON will be completed next year. It’s 1.6 miles long (same as Key Bridge). Zero pylons are in the water.
Estimated completion cost is $3.7 billion US.
Did I mention privately owned