Again, huge fires fueled by massive diesel tanks meant for emergency generators led to to the collapse. These tanks were set on fire sometime during the collapses or the plane impacts. The tanks were directly underneath a transfer beam that supported some 40+ stories of structure above it. Much in the same way the building in OKC fell after a key column was blown away, 7 WTC fell after this main transfer beam failed, leading to the collapse of a key major column. When this collapsed, the entire building went. Again, your nominal 2 hours of fireproofing burnt off after 2 hours. Rememeber this building did not collapse until late in the afternoon.
Yes - what you state is very plausible.
My thesis is that had other methods of insulating the steel columns been used (i.e., concrete encased steel columns, or wet asbestos), the buildings would have stayed up longer.
Those methods generally would provide 4 hours of protection, less in the case of the WTC bombings because of the inordinate amount of heat.
What happened as reported by the New York Times 4/28/1970 was that the construction companies were ordered to stop using wet asbestos (the were up to the 67th floor at the time with insulation), they immediately found a new technique (they couldn't use the older concrete encased steel beams technique at this point) which was "claimed" was just as good.
BTW - Today's Newsday (New York Long Island newspaper), carried an article about the collapse of the towers and mentions the effectiveness (or lack of), of the fireproofing.