The US has a unique location, resources (natural and otherwise), and form of government, all of which mean a decline of the US is potential, and potentially temporary. In the meantime, who's going to step into the #1 spot? Of course the French attitude will have all of our former trading and political partners stepping up and doing business with whomever takes the #1 spot, but many of our traditional allies and trading partners aren't going anywhere--Canada, UK, Australia, Japan, others.
The US may slip, but it won't be replaced by another, equal power. It will likely be replaced in that top spot by someone the rest of the world doesn't want there.
Even when they bluff that they can’t wait for the US to be replaced by, for instance, China. If they truly believed that the US was an oppressive imperialist nation, how could they even say the words to hope that China, of all nations, overtakes us? But I have seen Euro posters on Euro newspaper sites claim to want just that. It’s hard to believe that they really mean it and aren’t just displaying their reflexive anti-Americanism.
If the US slips, there will not be another pre-eminent superpower. THere will be many super-powers but no one completely dominating like the US did for 50 years after WWII.