Imagine a few hundred drones filled with explosives launched from a mile away from the Super Bowl. How are you going to stop them in the short time it would take for them to get to the stadium?
Imagine a few hundred helium balloons with baskets filled with explosives launched a mile away from the Super Bowl. How will you stop them? What about airplanes? Home made morters? Suicide bombers? Timed bombs? There are a ton of different things that could cause that sort of carnage.
I'm not sold on drones either, but suggesting that they would usher in an era of easy, quick terrorism strikes me as a slightly paranoid argument.
There is no society on earth that can prevent someone sufficiently motivated from causing mass carnage. Our error since 9/11 is believing that somehow, with the right mix of intrusion, paranoia and technology, we could stop it all.
Today more than ever we have nothing to fear but fear itself.