The dollar has been the reserve currency for a long time because (1) the US has been a big promoter of liberalization and world trade, but really most importantly, (2) despite various conflicts, the US currency has been consistent and stable for decades, and everyone who values stability is going to want to park their value in a currency that continues to be boring. So far, the US has been able to scratch that itch because it’s been the same government that has been predictably the same for nearly 250 years.
Now it doesn’t mean the world’s reserve currency won’t change over time but if it does, it’s because some other country or group of people have managed to establish themselves as equally boring for decades. And by the time that happens, the world will have already changed significantly already.
Really, the reason the US Dollar has held for so long is because the rest of the world has had a lot of instability (think world wars, governance changes, unions of countries formed or abolished, etc.). The US hasn’t even had a change in states in 60 years.