Every 20 years or so, the end of America is nigh — ever since the 18th century when, in France, Comte de Buffon fingered the country as a den of degeneracy while Abbé Raynal slammed its cultural poverty: America had not yet produced “one good poet, one able mathematician, one man of genius.” In 1987, in his book “The Rise and Fall of the Great Powers,” the Yale historian Paul Kennedy saw the United States on the road to perdition — this, four years before the suicide of the Soviet Union, which left America all alone in the penthouse of global power. Now, two decades on, it is the much-hyped “great power shift” toward Asia that will turn the United States into a has-been.