The only scenario in which the US Dollar would become obsolete is if a third world war occurs. If the economy of the US collapses as a result of the war, then some other currency will replace the USD. But the chances are quite remote.
Yes, it's almost impossible. Even after the war, the dollar may remain to exist perhaps only there will not be such a high rate compared to other currencies.
At the moment, the dollar is invincible. Even in the event of war with the dollar will be nothing. Let me remind you that the Americans are so smart, never fighting on its territory. The situation with North Korea once again proves it. Kim will never be able to make missiles that can reach America. He will be destroyed in advance.