There will be terrorist drone attacks in the US or Europe, but I highly doubt they will be at a 9/11 scale. The destructive power of a drone is nowhere near the power of a fueled jet, you can get close with a swarm, but that adds several new problems.

• getting high yield explosives is pretty hard. 9/11 stole the bombs on the day, which is much harder to respond to. You could try using drones to take out aircraft on takeoff, but that’s already controlled and monitored airspace.
• getting enough drones to be a large attack isn’t trivial even with readily available parts in small numbers. There’s already systems in place to monitor large orders of potentially dangerous materials like fertilizer or smoke detectors.
• coordinating hundreds of thousands of drones is actually pretty hard to do in a clandestine way. Sanctioned drone shows have problems occasionally while not having to hide and having to avoid countermeasures.
• Drone countermeasures already exist and are deployed for large events like the Superbowl. Target selection to be big enough but still insecure is also difficult. Security is more of a priority for iconic places and not all of it is theater.