Monarchys are legitimately a more unstable model of government. It's believable that with it's leadership gone, it would have a much tougher time recovering. There could be all sorts of civil wars or coups occuring in a vacuum that are far less likely to occur in a place like the US. There are actual reasons why people have moved away from that model of government... it's not just a comic book thing.
That said, if Coogler doesn't want to tell that sort of story he won't and is free to do something else entirely. It's merely a potential time to tell that sort of story IF Coogler wants to tell it.