No superpower moment lasts forever.
That said, the two largest obstacles we have to maintaining our superpower status are (a) our federal budget and (b) nuclear proliferation.
The budget's problems are significant, but even now, correctable--it would simply take a certain amount of domestic pain, primarily in the form of lower entitlement payments.
Nuclear proliferation is more concerning, but even in this age of relatively advanced and ubiquitous technology, assembling a nuclear weapon--particularly without a First World economy backing it up--is not an easy task. The countries that are not currently nuclear but which could easily go nuclear--Japan, South Korea, Germany, and perhaps one or two other members of the EU and Brazil--are not hostile to the U.S.
We may never again reach the relative zeniths that we reached in 1946 (nuclear monopoly and the rest of the world in ruins) and 1993 (collapse of the USSR), but it will be a long time before anyone truly reaches parity with us, even China or a completely federalized EU.
People have been predicting the fall of the American republic basically since our rise. It's easy to find things that are wrong with America, but that's not because we're significantly more flawed than anyone else, it's that we're more public about our flaws than anyone else, particularly when compared with potential adversaries. (The UK media and political culture also celebrates airing dirty laundry, for example, but so what?) That means that it's much easier to have a look at structural weaknesses in American than in China. That can lead one to believe that China has got things all figured out and America is a basket case. That's not necessarily true.