First, let's draw a distinction between America and the United States. The former is a nation of sorts, really more of a cultural empire. The latter is a nation-state. America is not a superpower. The United States is.
It remains one because it has ... wait for it ... the power. The US government has command of the planet's most powerful military, it controls much of the world's most powerful economy, it dispenses the world's most powerful set of benefits and punishments. As long as it possesses those assets, if you see them as that, it will be a superpower.
All empires and states do come to an end, of course. They carry the seeds of their own destruction, an observation that has been valid from the dawn of history and still is today. When the US as a superpower will meet its demise, however, is anyone's guess.