Uhm you do all know that this is a PR (and possible recruitment) even. DARPA is not serious about building a starship (America probably won't be around in a 100 years)
Why? The US is is more stable than any nation on the planet. Its the most adaptable, diverse....these are good qualities to have, and ones that many supposed "empires" that did not last didn't have. Even if the world is more united in 100 years, I'd expect the US to be a strong component. The strange idea some people have that the US is at the end of its infleunce is one of the most ludicrous I've seen, and very likely just wishful thinking ifrom certain parts of the world.