Both those things will soon become false - US economy will be eclipsed by China, and the process of de-dollarisation of world's reserves is accelerating. The fact that the US can print as many dollars as they want is one of the reasons for it.
[1] https://asia.nikkei.com/Economy/China-s-GDP-unlikely-to-surp...
Slower growth still means about 5% GDP growth per year, which is much higher than growth in developed economies.
The "demographic collapse" is a gradual decline in population over many decades, offset by vastly improved education among the younger generation entering the workforce. Just to give you an illustrative example, if three workers with 8 years of schooling retire from the workforce, and two workers with college degrees enter the workforce, is that a demographic collapse?
Yes, because, unfortunately, every economy depends on having workers willing to do occupations that don’t depend on high-level qualifications.
As China’s population ages while the pool of young workers without career aspirations shrinks, where are they going to find all the medical care-assistants, agricultural workers, and things like that? Other countries such as in the Middle East the “solution” was to import labour from India and SE Asia - I don’t believe that approach will work for China at all.
China's economy was projected to overtake and eventually eclipse the economy of the USA. China is currently staring down an existential population collapse. China's also a net importer of both energy and food.
Pointing to BRICS nations dropping the USD for trade demonstrates a fundamental misunderstanding of what BRICS is.