>Why would slumping or buckling but not-quite-collapsing be a functionally better outcome than complete failure?
Well for starters, there's a better chance of less (or ideally no) people forced to get their feet wet and go missing or die from drowning or hypothermia.
The specific point I'm making (in the sentence following the one you quoted) is that no, there is not a better chance of those things. If a bridge of this size is going to fail in any significant way, it is going to lead to loss of life.
The broader point is that attempts to blame the bridge designers here are misplaced. It really isn't reasonable to have hoped that they could design a structure that could cope with this kind of failure, within the cost constraints that they had. And cost constraints were a real thing for the designers of this particular bridge.
Well for starters, there's a better chance of less (or ideally no) people forced to get their feet wet and go missing or die from drowning or hypothermia.