Interesting, what might be the reason for its success despite being a presidential system?
The political independence of the military, its culture of unwillingness to step into politics, and the taboo on the use of the military for policing on American soil, is another key component in American stability vs the rest of the Americas.
Whether this holds up for another 5 years is anyone's guess. My family is from Argentina, and having lived there myself I see the US sliding in that direction. One very telling thing is the increasing justification of political violence that would have been unthinkable a decade ago. However, America has been through periods of political violence before and has not degenerated into a dictatorship, although it's come close.
Instead, the police militarized. Then there is the National Guard(?).
However, I also lived in countries in Asia and Latin America where the actual military plays a major role in policing and putting down protests. Now I live in Portland, where the militarized local police put down protests, and I can say that the qualitative difference between those situations is that the local police do not represent the government of the country. They still answer to the courts. They can't throw you in a black hole without legal representation. They don't have the true power of the state behind them. They may be fucked up as hell, but they aren't the army, in the sense that the actual military is sent in to suppress uprisings around the world.
That was my point about what allowed America to escape the tragic cycle of coups that has befallen most other presidential republics ... the taboo on the military engaging in domestic policing. No other country in Latin America has that taboo, and as a result the army in each county functions as a bludgeon by whoever takes absolute power -- usually fascists, but in odd cases (Venezuela, El Salvador), leftist totalitarians.
It ensures a two party system which makes the government lean center (on both sides) and be very stable.