If they wanted to hit New York for strictly economical reasons then they would have flown all three planes into Wall Street. They hit the World Trade Center because it was a huge monument to the American way of life, it would kill thousands of people and because it was a huge arena for commerce, not just because they thought it would destroy our economy.
Besides, our economy is so huge, it's going to be hard to be interrupted by terrorists. What happened after the WTC attacks, besides a bunch of media bullshit? Nothing. People kept of buying stocks, people kept of buying durable goods, people kept of buying cars, people kept on buying houses, etc. In the grand scheme of things the WTC attacks aren't even a blip on the economic radar screen. Sure thousands of people got squeemish and jumped out, but there were thousands of others ready to jump in and take their place.
Attacking an economy as huge as the one of the United States of America just isn't a viable terrorist plot. Sure they'll take every chance they get to knock a little chip off as collateral damage, but it's not going to be their main goal.