At least in America if you’re not a fan of the Trump administration we’ll have another leader in a couple years. You can’t say the same for China.
Brain drain from the US will be a dispersion that will hurt America but not meaningfully help any single country directly. At least in the short term.
But China has a lot to gain from a weakened America simply by being the best positioned, most aggressive secondary power.
Of course you’re right that an individual emigrating from the US to China is economically net-negative for that individual except for in very unusual circumstances.
People don’t have to move to China for them to benefit.
I think the problem is that the US presidency is too powerful, it might be better to turn the presidency into a ceremonial position. It shouldn't be possible for the president to do all this damage.
“[Political factions] are likely, in the course of time and things, to become potent engines by which cunning, ambitious, and unprincipled men will be enabled to subvert the power of the people and to usurp for themselves the reins of government and destroying afterwards the very engines which have lifted them to unjust dominion.”