Your premise only works if all nations are the same, all goals of all nations are the same, and all nations are all peaceful and allied, no nations ever compete, and there are no military threats ever. That isn't true of course. You'll notice I didn't say anything about one nation being good or evil, rather, it's about the actual reality of the situation: the US and China are going to be strategic competitors indefinitely, not pals, not close allies. The same holds true for the US and Russia. Nothing is going to fundamentally change about that, period. These nations are permanent rivals, so long as their stature is what it is.
China has openly touted that their intention is global dominance (in all regards, including military, economic and technology). They haven't been shy about it for a long time. Given their economic and military scale today, you have to take that very seriously.
The US spends anywhere from 100% to 150% more on its military than what China does. That's mostly a difference in salary expenses in the US vs China. In properly adjusted terms, China is already matching or exceeding the US on military spending right now.[1] What will it look like when their economy is another 50% larger? How are the smaller, weaker nations in Asia going to deal with China if they're already annexing territory the size of France in the region? And then, in theory at least, their military is going to get drastically stronger.
A large amount of future global economic expansion will occur in Asia. So when you combine those two points, again, you have to treat the rise of China as a very serious matter for economic and military security if you're a superpower with outsized global interests as in the case of the US. That also goes for countries all over the globe that deal with China locally, and especially in Asia. Two of the best and most prosperous US allies are Japan and South Korea, who are also two of the world's largest economies. The US has an enormous vested interest in their well-being.
It makes no sense - no matter what you believe about the US or China - to pretend that the US should just treat China like they're another random smaller nation of no concern or special consideration. The exact opposite is the case. The US and China are going to compete at most things and it will split the world in half.
[1] https://www.bloomberg.com/opinion/articles/2018-05-25/china-...