You don't get it. It couldn't have been spent on something else. If the day before 9/11 Congress had passed universal healthcare, free college, and basic income, they still would have spent this money on a war. Are you saying they could have said, oh we got attacked, let's estimate the cost of a war and put it into healthcare?
“Reality is unchangeable, so we can’t know the unknown”
So what? It definitely could’ve been spent better than decade’s killing hundreds of thousands of civilians.