I can’t say I particularly disagree, however I think you’re overestimating the moral character of states in general. If US hegemony erodes over a “century or so” I think that is a manageable course of events rife with opportunities for building a better world, as you say. If, on the other hand, the US were to suddenly become incapable or unwilling to fill its role as global hegemon, the resulting power vacuum would undoubtedly effect chaos.
I hope for a graceful retreat from imperialism into some sort of international socialist utopia… but history isn’t exactly reassuring.