As long as they can continue making interest payments on the debt then what's the problem? It's all relative.
So nobody is ever concerned about debt; you just say screw it let someone else worry about it, usually future generations. The value has to come from somewhere, and in this case its from the debasement of the dollar.
The relevant question is can the US continue producing goods and services that are desirable on a global scale, one of which is a trustworthy society where you can mostly depend on the judicial system keeping things in order, for better or for worse.