In a recent game I played, the civil war never triggered for the US, and they only had really high militancy, which triggered so many revolutions! First it was a bourgeois dictatorship, then democracy again, this repeated about five times. Then when a bourgeois dictatorship was overthrown by reactionaries, it became and absolute monarchy! This regime lasted about 20 years, which I think was the longest period of stability for America in that game. It then collapsed a few more times, becoming democracies followed by more bourgeois dictatorships. They fell to 8th place by about 1850, and never returned to GP status, then were a secondary power until 1860, then rose occasionally 9th place, sinking to 21st after each revolution. All the while, Mexico became 3rd GP by about 1880, and slowly gobbled up all the land West of the Mississippi! It stayed a stable democracy and became the no. 1 place for immigrants for a few decades, I think by about 1920 its adult male pop was something like 18M, Americas was only 3.1M!
So, if you want America to plop, just somehow stop the Civil War from firing.... or use other methods. But it's not inevitable for it to be a GP forever.
(The same game also saw the spectacular fall of France, which after 1860 never had a regime that lasted more than 3 years or so. by 1890 it never rose above 23rd place, and was eaten up by a very powerful Italy and Germany. Austria collapsed as well, with Slovenia and Croatia being lost to Italy, and it never really being more than a secondary power after 1870, even though for a few years in the 1840s it had been the no. 1 industrial power). By 1936 the great powers were the UK (me), Germany, Japan, Mexico, Peru, Scandinavia, Italy, and The Russian Empire. Spain and Portugal did very well in this game, holding 9th and 10th place respectively, because of me using my ample cash reserves to industrialize them both, which meant for once Portugal colonized its African colonies! One of the more interesting games i've played! Even though I was the UK, I barely touched other countries other than protecting investments by sending troops to countries overrun by rebels)