You arm-wave but provide no quantitative rationale. Plenty of heat in a closed-box fireplace burning 50 tons of kerosene. Enough to heat the structural steel to 30% or less of its room-temperature strength. All you need to do is reduce the strength margins to zero for a catastrophic collapse. No ordinary building would have so much high-energy-density fuel to combust. There is some basis for thinking that some of the aircraft aluminum contributed to the combustion.