You forget that there are houses below the bridge, that could have been crushed. Not exactly a place for destructive testing.
If the bridge is rotten, you have lot fewer victims.
You want to do pre-emptive repairs? OK, the bridge ought to be fine. We can then proceed with the destructive testing.
You could do the testing with something like 110% of the typical max traffic load. A rotten bridge would then collapse within 6 months of it's "natural collapse". Only things you do is to make the collapse more predictable and you save lives.