Was September 11th Foreseeable?

In the 1970s a U.S gubmnt committee studied the potential for terrorism. One of its conclusions:

Committee members identified commercial jets as a particular
vulnerability, but raised concerns that airlines would not pay for security improvements such as tighter screening procedures and routine baggage inspections.

``The trouble with the plans is that airlines and airports will
have to absorb the costs and so they will scream bloody murder should this be required of them,'' according to a White House memo from 1972.

``Otherwise, it is a sound plan which will curtail the risk of hijacking substantially.''

Quite clearly the committee was concerned about hijackings, not the use of fully fueled passenger jets as missiles. But equally clearly the committee understood there would be lobbying to deter the installation of increased air safety procedures.
[Thanks to BenS for link]

And now we face the same situation with the threat of shoulder-held rockets [registration required] and airliners:

Rand, a nonprofit research firm, studied military-based antimissile
technology and concluded that adapting it for commercial airlines would be prohibitively expensive and that the equipment would be too difficult to maintain.

...The report acknowledged that the loss of even one commercial jetliner to a shoulder-fired missile would be significant, estimating the cost of the aircraft and legal settlement of numerous deaths at $1 billion. The cost could grow to $15 billion over several months from a single attack, if travelers were then reluctant to fly...

There have to be a lot of conditional probabilities that go into this sort of cost-benefit analysis!

Also notice that questions like these, like it or not, require making a judgement about the value of a human life.