How long does the US economy have before it falls?
Its not going to 'fall', instead we will see decoupling (as the relative importance of the US economy falls). The important issue will be "how will the US respond?". One would fear a right wing reaction akin to threatened imperialism where the power shifts encourages even more aggressive foreign policy
The curtain, a funeral pall, Comes down with the rush of a storm, While the angels, all pallid and wan, Uprising, unveiling, affirm That the play is the tragedy, 'Man,' And its hero the Conqueror Worm.