I disagree. If a scenario was plausible or likely, it would have actually happened at some point in thousands of years of human history. If across a sample that consists of the entirety of human history a scenario did not occur, that is strong evidence that it is not plausible, and evidence (albeit weaker) that it is not possible.
Keep in mind that logistics was horrifically difficult for large empires thousands of years ago. Even with Rome building roads for their chariots leading from Rome to everywhere in their empire, it still takes a long time to get that horse with your imperial decree from point A to point B. Without a means of reliable long-range high-speed communication, you de facto
need high local autonomy. That means local governors with nigh-absolute power within their sub-borders, which does not breed loyal vassals.
Then add on that in order to
stop such a local governor from ignoring your orders, you have to march your army across an entire continent, with your rapid-response being horse speed.
The simple fact is that for most of human history, the world was simply too
big to conquer. You'd be utterly unable to actually exert your influence over the entire country, and with the upper brass of your empire being used to their total authority your court is going to be full of people ambitious and corrupt.
As technology advanced and we gained the ability to effectively communicate across these sorts of distances, what's the first thing we saw? Imperialism. Countries like Great Britain and Spain and Portugal went all around the world, growing empires. I'd argue that it's only fortuitous coincidence that European matters led to mass-independence from the colonies during moments of weakness in their mother countries and Imperialism in general going out of style.
The Elemental Nations doesn't have the telephone, sure, but it's also fairly small overall. Leaf can maintain relatively tight control over matters on the local scale, preventing authority bloat in the local governors, and if Hazou doesn't die then telephones are on our list sooner or later.