People get very nervous about sole superpowers as well... especially when people think they are capricious and prone to unilateral action.
The US Navy may not have the number of assets that it once did, but it's still orders of magnitude beyond any other Navy.
You can drive yourself batty envisioning scenarios, and you can drive yourself bankrupt preparing for scenarios, and you can drive yourself into a world of $#!t trying to preempt scenarios.
A total US pullout from the ME and South/Central Asia is unlikely, and envisioning scenarios based on that assumption is a largely academic exercise. Envisioning scenarios based on excessive sinophobia or russophobia is also not terribly productive. We can't assume that we must have military dominance over every strategic area of the world because if we don't, somebody else will... the cost of trying to maintain that dominance will choke us far more surely than the possibility of having to share influence.
Certainly it makes sense to envision and prepare for scenarios built on the assumption that the US will no longer be a sole superpower, as the US no longer has the economic wherewithal to maintain that status.
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