I feel like I'm not communicating my point well and you're misunderstanding. Decline is inevitable. Not universal decline though. I believe we'll move forward as a species, but that overall progression is made up of lots of groups constantly declining and improving at different paces and times.
I think you don't understand what I'm saying because you said e.g. "others like China can rise" - my point is China has already risen, and fallen, and risen and will continue to do so: https://en.wikipedia.org/wiki/Golden_ages_of_China. Just like the US will. And the troughs will be tragic compared to the peaks.
This is what long term empires do. They rise and fall and rise and fall, and that rises and falls include wars, famine, disease outbreak, advances in war, science, tech, health etc.
I feel civilised societies have said exactly what you've said since the dawn of man "we're civilized, we've moved beyond incivility now" when in reality, they were just in one of the many times where their society just so happened to be leading the way.
Sorry, because I know you don't believe this, and you want to believe "we the people" can stop change this natural cycling, but it's a feature of relativity. Ultimately, you're saying the same thing optimists have been saying for millenia, and here we are, war, hunger, famine is all still happening. Same stuff we've been doing for millenia, just with fancier tech.