The moment you stop being an empire, the dollar stops being the world's currency.
The moment the dollar stops being the world's currency, the USA default on their national debt.
The moment the USA default, your quality of life goes down the drain.
A significant portion of your standard of living is linked to the massive economic advantage of being the world's first superpower. Once that goes, so your standard of living is going to drop.
The standard of living for the average American has been dropping steadily since 1976, at least as measured in real income. We've been outrunning a massive correction on debt for even longer. If there must be a collapse (and really, there must. That which cannot stand will not stand and all that), why not combine that with the added benefit of ending our Imperial ambitions?
Also, isolationism works only until the world comes knocking to your door. Ask Bush.
Actually, a very solid argument could be made that 9/11, which i must assume you are alluding to, was the result of Imperial Policies. The Mujahadeen that trained Bin Laden were funded by American Imperialists, the inspirational moment that triggered his desire to destroy skyscrapers was seeing Americans bomb towers in the Middle East, and the hijackers trained in American piloting schools and hijacked plains from American airports... both of which were made possible by an Imperial border policy that views all members of the Imperial Provinces (in this case, Saudi Arabia) as having full rights in Rome, so to speak.
Tell me exactly how Bush, either Bush, was following Isolationist policies?
I'll mention one other benefit, a purely personal one I suppose, to getting out of the Empire business. I hate hearing people... foreigners under the Country model or Provincials under the Empire model... bitch about the American Empire, and how much they want us to just Go Away and Leave them Alone. I LOVE giving people what they ask for, especially if I know they'll bitch about getting exactly what they asked for. It isn't quite Schadenfreude, but some species of close relative. I'm SURE the Germans have a word for it. I mean, fuck: They've got a measurement based on mosquito dicks. Their dedication to producing awesome words is inspiring.