Since countries like China are carrying on in space exploration, the argument can pretty easily be made that if we (the government) don't continue our own space program, we would be shirking our defense duties.
Even assuming China collapses into the stone age, space-specific research is still a defense priority. Basic science, carried out correctly, *always* has numerous unforseen benefits, most of which have defense applications. It would be criminal to allow the US to stop progressing. Also, the best way to assure that the United States cannot be destroyed is to make the United States unbeatable, and the best way to do that is constant expansion. And the Earth is full, noplace left to conquer.
A civilization either grows or declines. "Static" just doesn't really happen.