Meanwhile, the West has its own implosion well underway... but, unlike China we have immigration. The U.S. population is still growing thanks to immigration, as is Europe's, even though both regions have replacement-level or below-replacement birthrates by natives. Immigrants tend to have higher birthrates however.
Still, I would prefer to "quotient out the money", and say either the demand is met or it isn't? To the extent it is met, perhaps there are imports of material goods or domestic labor (nurses?)
I don't think it makes sense to worry about "domestic debt" in the same way, given the non-liberal political economy. Of course that doesn't mean there aren't limits, but the limits maybe would take the form of e.g. rising discontent and decreased productivity if it feels like an old-people-focused service economy with nothing in it for the workers.
You can't just blindly apply the economics of traditional liberal economies to a 21st century high tech illiberal economy.
Now the one child policy has been relaxed, but it will take a while before extra kids are available to work, and at this point it’s sort of a cultural norm to have one kid (plus prohibitively expensive to raise two kids and provide them decent resources).