I don't see most here saying that the past performance of the US guarantees future results, just that they aren't betting against good future results.
Having said that, it would seem the clear answer is right in front of you. You probably don't put all of your assets into a single investment, so if you're concerned about the future of the US you definitely shouldn't put all of your assets solely into US markets.
Diversify. Invest in other countries. Find global funds (or maybe, global funds that exclude the US) and put significant sums in those. Don't even try to pick which countries will and won't do well in the near future, just invest in all of them. Surely not every country will perform poorly going forward (though, honestly, if the US collapses in some major way that will have a huge negative impact globally). And if the whole world does collapse - financially or otherwise - well, there was nothing you could have done to secure your situation anyway.
