Also I'm not sure that foreign markets would be that thrilled by a US collapse either, at least not immediately.
Assuming you're talking decades away, it usually all comes out in the wash.
Now, where you should really potentially worry is if you were retiring imminently and needed to pull out a bunch of money to make that happen. But if you're retiring in 20 years or whatever, and, say, the S&P halves next year, is it really, in the scheme of things, _that_ big a deal?
If you could time it perfectly, you could come out better by selling now and reinvesting after the crash, but bear in mind that you probably cannot time it perfectly. People were predicting the 2008 crash imminently from about 2004 on, say, whereas the dot-com crash went from dark mutterings to chaos in a year or so. These things are very hard to time.
if the upswing doesn't come our lifestyles are all screwed anyway