What would be so bad about the US losing its supremacy in the world?

What bad things would happen if some other currency became stronger than the dollar? Would it be bad for everyone, or only for Americans, while lifting up people in other parts of the world?

What bad things would happen if we scaled back our military and played nice with the other countries as peers and equals?

In general, why do so many Americans, even those on the left, view the end of US supremacy as a bad thing? I mean, if the US is losing supremacy by declining, then the problem is the decline, not the loss of supremacy. But it seems like the loss of US supremacy per se is a cause for great concern, even fear. Why?