Home heating oil can burn hot enough to weaken steel beams holding up a bridge, through the asphalt and concrete road surface above. There was a fuel truck explosion and fire in Bridgeport, CT in 2004, and that’s what happened. Heat would tend to escape above, but still, enough went down through the asphalt and concrete to weaken the steel beams enough that the bridge was drooping and had to be entirely replaced. And this was a *new* bridge, of all bridges in the state, one of the ones most likely to survive an incident like this(source: CNN).
Focused the way it would have been at the WTC, and combined with a significant kinetic impact, I’d be shocked if jet fuel couldn’t critically weaken the steel beams used in the construction.
Add in that the specifications called for survival only against a significantly smaller and slower aircraft, if anything we should be impressed that they stood as long as they did after the impact, rather than be at all surprised that they would have gone down in the end.