You think the US government is going to shrink down enough to pay.... how much is it now ? Or the US will "grow" out of debt ? You know it's not possible by now.
What's wrong about being upfront about it and tackling it head on ? It will hurt less than if this bubble pops on its own accord. Or so I believe.
I'm pretty sure if someone was willing to let the US default, they would suffer politically, but don't you want someone who will think long term instead of kicking this can down the road ?