This is a theory, not a really good one, but lets take it at face value, starting with the end of imperialism. Given that USA continued it’s imperialism after WWII while WWI is the beginning of the end of imperialism for Europe. The British empire stopped existing in 1998, but I’d say the end of British rule over India in 1947 was the peak for the end of European imperialism. This had nothing to do with USA.
Decolonization: This didn’t happen until the 60s and only after fierce resistance from indigenous people withing the colonies. While the USA held a powerful seat the UN that called for decolonization, the fact that USA held its colonies (e.g. Guam and American Samoa), and that it took fierce colonial wars before liberation, I say indigenous resistance had way more weight than any USA influence of the matter. Portugal didn’t actually relinquish their colonies until their dictatorship fell in a socialist revolution in 1974.
Rise of democracies: Again did not follow USA occupations. Staying within Portugal, The Carnation Revolution would certainly be the first time the USA backed left leaning armed forces against a right wing dictator. And we know they didn’t, USA stayed out of that one. Greece on the other hand didn’t became a dictatorship until after it had joined NATO. Now this dictatorship was no friend of the USA, but that only further proves how little USA hegemony had in preventing these dictatorships and in the rise of democracy. I would actually give the EU more credit here. Speaking of which,
the EU: I honestly don’t know how you can give USA hegemony credit for this. It started as a free trade agreement, USA didn’t start doing anything similar until 1994 with NAFTA. Stating USA hegemony here is frankly a little insulting, insinuating that Europeans cannot take autonomous decisions regarding their own affairs.
The only think we can definitely thank the Americans for is demilitarization and the initial buildup of infrastructure.
But I think you get the point here. Attributing all of these to USA hegemony is a very simplistic—and frankly wrong—view of history. Honestly at this point I can’t tell what you consider hegemony, if USA had hegemony over Europe since 1945, how did it not have hegemony over Argentina during the dictatorship?
Decolonization, including the dismantlement of the British Empire, was an explicit policy goals of the US dating all the way back to at least WWII. One of the signature examples of this orientation—and the first clear assertion of American hegemony even over Britain—was the Suez crisis.
> Rise of democracies: Again did not follow USA occupations.
The democratic governments of West Germany, Italy, France, Belgium, the Netherlands, Denmark, Norway, and Austria were effectively installed by the US and UK at the end of the Second World War. Without American involvement, none of these countries would have become democratic; they would have either remained fascist or become communist.
(This was not the case for e.g. Argentina, Turkey, or Portugal.)
> the EU: I honestly don’t know how you can give USA hegemony credit for this.
The core concept of the EU is the goal of establishing a “United States of Europe”. In other words, it is an attempt to emulate Europe’s hegemon, and it arose almost immediately after Western European democracy was installed by the Allies at the end of WWII.
That doesn't imply that the US had any hegemony or control over Argentina.
> Don’t you think the UN deceleration of human rights helped with minimizing conflicts, the rise of democracy, the end of imperialism, decolonization, demilitarization, buildup of infrastructure, the European Union, etc.
Why did all of these things happen in the first place? American hegemony. You can tell because Europe tried half of the things on that list after WWI but they didn't work.