In China's case, the population decrease is actually a positive for them since they are primarily an exports-driven economy. A lower population means investing into automation at an extensive degree to retain the same production levels, without the need to feed that much of a population. And if China really needs an extra labour pool, they have no qualms doing the Middle Eastern playbook and bringing in tons of workers from low-wage countries to do the dirty jobs - in fact, they already do that with Africans.
Russia is in trouble though, but given that their industries are slowly being eaten by Chinese conglomerates, they are a has-been now.