Bad enough that car/truck traffic is stopped.....boat traffic, too.
They say a good weld is stronger than the steel itself. It means they could fix this particular break and get stuff moving again, assuming an inspection turns up no other problems. It would at least buy time while the new bridge is being built.
Also, a bridge should last more than 50 years. I used to live by San Francisco in the 60’s. All the bridges we used are still standing and doing fine.