No government could be ... "so dastardly as to kill 3000-odd of their own civilians"
That's how they were hoping the world would react.
btw ... the US Govt does not regard 3,000 as a high death toll. In the US, someone dies on the road every 16 minutes. In the past 65 years, the smallest number of people killed the road in the US has been 32,479, in 2011.
So, since those 3,000 people were killed by "foreign aggressors", over 330,000 Americans have been killed by other Americans crushing them under their cars.
But those 330,000 deaths are an irrelevance in the public perception. Americans get hugely worked up about stopping perceived terrorists from "killing us". But they are not so worked up about killing each other, every single month.
So was a death toll of 3,000 inconceivable? No, it was a drop in the ocean. In September 2011, more people were crushed to death under cars than died in 9/11.
So it didn't significantly increase the number of Americans who died prematurely. But it did significantly increase the willingness of the whole world to support military action against a perceived enemy.
So, IF 9/11 was the result of a plot ... would the authorities sacrifice 3,000 lives? Yes. Because, in statistical term, 3,000 is a very small number.