Even cockpit doors are going overboard. The 9-11 attacks were about attacking buildings with flammable liquids. A gasoline tanker and a small bomb would have killed 10× more people and been cheaper.
Getting a gasoline tanker close enough to cause sufficient damage (i.e.: inside) would be hard; not only does it have to get close, it has to get inside. WTC famously had "10,000 pound planters" positioned around the buildings to stop vehicles from getting too close, and IIRC rather aggressive security preventing unwanted vehicles entering; they learned from the garage bombing about a decade before (just didn't extend vertically). Hijacking a just-took-off airliner was the most efficient & surprising way to transport & insert the liquids.
Cockpit doors are a cheap sensible solution. 10,000 pound planters worked until somebody figured out how to go over them.
A gasoline tanker and bomb would make a fire. As with any other vehicles on the road, they crash and catch fire on a regular basis without taking down sky scrapers.
Hollywood representations of what gas tanks do when ruptured are inaccurate.
Yes, from a plane, which first made massive holes in the structure, followed up by the fuel from an airline that carries more than one tanker truck worth of fuel being deposited into the center of the building.
Good luck getting a tanker truck into a modern skyscraper's lobby. They've all got anti-truck posts concreted in around the perimeter to prevent just that.
