Perhaps there's another answer...
sub6
It is just my (strongly held) opinion.
But I agree it is not the majority view.
Decapitation was probably not a primary AQ motive.
My opinion is based on still-unproven allegations regarding the complicity of national governments in the 9/11 operation. If you accept the view that 9/11 was solely an AQ operation with no other governmental involvement (Iraqi, Iranian, etc), then looking at 9/11 primarily as an attempted decapitation makes little sense.
I think folks will be arguing about the origins of 9/11 as long as the JFK assassination.