The outcomes of civil wars is widely acknowledged by both state practice and opinio juris as being a legitimate factor in the determination of sovereignty over a territory. If you don’t believe me, ask the Confederate States of America and the Republic of Vietnam about their experiences and get back to me.
There is no “morality problem” because there is no issue of morality here. Morality is not a factor in international law.
Dedollarization and constant US imperial overreach are the two factors which are most likely to break US imperialism in the mid to long term.
American economic dominance is propped up by the ubiquity of the dollar in general trade as well as the Petro dollar. In general trade, more and more countries are pivoting to trading in their own currencies or Euros and Yuan and Rubles because of the destruction of confidence in the US dollar as a neutral reserve currency due to recent sanctions against Russia. In terms of the Petro dollar, the trend of decarbonization means that oil will be a less critical commodity over time and even now we see the likes of Saudi Arabia agreeing to sell oil to China in Yuan. Without US dollar dominance, America will not be able to print as many dollars to service its debts, which will lead to either inflation or debt default.
America, like the UK and France before it, doesn’t have the ability to fight all of its repressed imperial subjects at once. The cracks are starting to show at the US giving up against the Houthis in Yemen. The US and EU has also pegged its military prestige to the war in Ukraine, which is also starting to turn. Not only are they taking a reputational hit with every picture of a burnt out Abrams or Leopard, but lesser US allies are also starting to see that full US support doesn’t guarantee victory. Even within US policy circles there is some acknowledgement that defeat in Ukrain could lead to some sort of Suez moment for the US and NATO.