The USA doesn't have enough nukes to wipe out anywhere near 90% of Russia's population, let alone China's.
Our Nuclear Forces aren't modernized by any standard, and the new upgrades in the future aren't going to increase capability.
The same can be said about their ability to demolish 90% of the US population. I do believe the US has more modernized nuclear forces than anyone else though. I’m almost 99.99999% sure a good chunk of Russian nukes haven’t even been serviced or looked at since the Cold War era.
I think China will defeat the USA economically ultimately.