If demand for the dollar dropped then the dollar would be devalued. Making imports more expensive (and exports cheaper). A mild devaluation might actually be good, after the initial pain (if you want companies to make more things in the U.S. instead of bringing them in from abroad it would help that, without starting a trade war over tariffs)
I don't believe that there are any definite plans to replace the dollar as an international reserve currency.
Of course if Trump does something crazy - like default on debt repayments - who knows what will happen. (Not saying that he will, but can we be sure he won't?)
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