Still, a USA which is just a moderately bigger Brazil with maybe 10x more talent at the far-right end of the intellectual spectrum isn't much to be proud of. The institutional degradation would be akin to a country like South Africa, where they have plenty of extremely talented people which can't get anything done since the government is controlled by extremely corrupt and incompetent people.
So, the Elon Musk of this world will still occasionally immigrate to the US at very high rates, but will find it much harder to be successful because they are surrounded by morons.
I really don't see any possible reality where China is not the absolute power in every domain, except perhaps cultural/entertainment but who cares about that anyway. While America has multiple unalterable existential risks, China as far as I can see has either one or two. One is the fertility rates (which the government is now trying to reverse so good for them), the other is arguably a return to outright Marxist/Maoist/Communist doctrine. I don't think this latter one is an existential risk though, that's assuming it's even true. A country can always reverse paths again. The only real existential risks are demographic change (especially of the racial-ethnic kind) and possibly climate change.