Hardly the only two countries. In the US it's only masked by immigrants. Fertility is even coming down in most parts of the third world.
It's mainly attributable to women's improved education, career prospects, and access to contraception, plus declining infant mortality. Every single one of these factors is a good thing, but the combination of them will lead to a global demographic crunch over the next century.
I would have thought that space debris is deadly no matter if it's made of wood or metal. If something comes at you at a few kilometers a second, it doesn't really matter what material it is.