Did they need the real estate under the World Trade Center to establish a caliphate?

Which was why it was widely considered a dumb move on the part of Bin Laden. It attracted all sorts of unwanted attention, that ultimately led to the the core group losing all relevance. The other al-Qaeda franchises might be doing well, but the people who pulled off 9/11 (and their successors) have virtually no influence, on top of being hunted by the US.