This is mostly an opinion peace, written after the fact of how history played out and vastly overestimating the influence of the US decisions on the global picture.
Since we only have one history, you cannot claim that the world had been so great (even that is an assertion made on unclear parameters) because of the US; we simple lack quantitative comparisons. Maybe the world could have turned out way better if the US had taken different decisions on various issues? This story/opinion is a typical post-hoc ergo propter hoc logical fallacy.
The world is a complex and chaotic system, meaning an abundance of parameters (countries, people, technological innovations like the internet, ideologies, wars, religions, economics) can influence its fate so drastically that it is unpredictable (chaos theory).
That all being said, I somewhat agree with the conclusion, that we have to at least try to affect the parameters we can positively. It would be a shame to see the world turn worse again not because of factors we cannot influence,
[…] but simply because they chose to stop trying”
If we stop trying to better ourselves, we are not increasing the odds of making the world better.