Dude I completely disagree with your premise on the US however i respect your opinion, but yes the world would rally against China if they become an aggressor, even if other countries hate the US, the enemy of my enemy is my friend, and any country being invaded by China would happily accept military support from the US, and other countries would gladly side with the US and other first world countries out of fear of being invaded. A mutual defense against a Chinese aggressor is the logical choice vs each country dealing with a Chinese aggressor on their own. You have a right to think of the US however you want, but drop your bias for a second and think of the logical approach to this situation.
Also, I do not worry at all about the BRICS, in fact most of the countries are in opposition to eachother in one form or another. you do realize that India and China hate eachother and are in fact in the middle of military border disputes, Russia is a dying country, with birth-to-death rate of 0, and their entire economy is built around oil and gas which in 25 years will be almost entirely replaced by renewable energy sources.
To quote Wikipedia on the BRICS nations it says, “The existence of the BRICS grouping does not signify a formal or informal alliance; there are multiple economic,
territorial, and political disputes between the five governments.”