Those buildings were really not designed to protect against that level of failure.
This is incorrect. The twin towers was the first case when the design explicitly considered impact from the largest jet airliner of the time (DC9) fully loaded and the subsequent fire. In fact, we know that two separate studies on this were carried out at the design stage. The conclusion was that the fire would kill but the towers would not collapse.
I take no position on whether additional factors contributed to the collapse. There are so many fanciful theories floating around that it is almost impossible to separate truth from fiction. What I will say is that the investigation was unlike that into any other major failure I have ever witnessed. With both towers failing critical design criteria, one would expect all the evidence to be carefully preserved and a minute examination of the debris to find out why. This is not what happened. Indeed, the initial investigators (from AISC) were not even permitted access to the site for a month, by which time the evidence was already being shipped overseas as quickly as possible. By the time NIST was involved nearly a year later, the vast majority of the steel and other debris had been disposed of.
Given the American penchant for launching lawsuits at the tip of a hat, it is truly amazing that the firms responsible for the design and construction of the towers have never been targeted in civil suits. It would seem that anyone with relatives killed by the collapse would have a prima facie case.