The US Navy is a major player here, having recently seized Venezuelan and Iranian tankers on the basis of violating sanctions. Only in retaliation did Iran begin seizing boats. This undermines the thesis that the US Navy is somehow protecting the stability of global trade. They're actively destabilizing to advance Washington's hegemonic interests.
> The thing about Zeihan’s arguments is that you can’t really argue against demographics.
You can absolutely argue against this. It's called automation. Needing millions of young healthy bodies to push machines around is increasingly outdated. A highly technical population with "bad demographics" will easily outperform a 20th century population with "good demographics" in the 21st century. Not to mention high population growth (required for "good demographics") is increasingly unsustainable due to natural resource limitations.
Zeihan is frankly a crock. He makes pretty maps and bold claims, but his evidence-based analysis is incredibly shallow. His MO is selling books: geopolitics as entertainment, not providing useful analysis.
You are much better served by listening to experienced realists like John Mearsheimer or Michael Hudson, rather than Zeihan's pop-geopol junk.