Zeihan as a source? That's... optimistic. More entertainer than analyst.
Premature to write off 1B+ on the birth rate issue. Remember peak oil? We adapt. Alternatives exist (and will).
US good demographics via immigration, shielded by deep water, internal water ways... fortress america. Shale revolution for energy autarky. Food security. PRC not.
Reality:
PRC growing from current roughly parity to 2x-3x larger skilled workforce vs US in next 20-30 years... who will be in workforce 2060/70/80s (this is already baked in from past 25 years of births), i.e. medium/long term of our and most of our children's lifetimes, long enough that hard to extrapolate after. Also the highest concentration of automation in the world. Maybe this is gap AI can plug, but in absence US with even immigration cannot come close to PRC simply minting roughly OECD combined in just STEM. Meanwhile PRC pop trending to 800m by 2100, i.e. 600m less mouth to feed and fuel but remaining 800m mouths that's 60/70/80% skilled workfroce vs current 60/70/80% lowskilled/peasants is a complete different strategic competitor. PRC declining population, but better workforce = most optimal competitive demographic trend to compete with US. Reminder that JP/SKR increased economy 10-20x post <2 TFR simply by upskilling their workforce. PRC current "only" has 20% high skilled workforce, them moving to 60%+ of advanced economies = decades of stupendous amount of high skilled demographic divident to extract.
PRC dredges shallow shores to build plurality of most high performing ports for global trade.
PRC infra/waterworks connecting internal waterways gives it highest utililized internal water transport system in the world.
PRC capacity for renewables has higher ceiling than US shale, cheap shale is not endless, extracting tapped out permian is projected to increase, $70-80 breakeven in 20 years might kill economic competitiveness vs other producers, vs hard to project anyone competing with PRC on industrial renewables.
PRC developing global strike to pierce fortress america... which really is misnomer for expeditionary america, i.e. technology that forward deploys advanced military (relative to others' projection powers) to bottle up others within their shores so they can't reach US. Remember how the US experiment could have ended off CONUS shores by wooden british boats if it wasn't for French intervention. Vunerability of CONUS is product of geography mediated by technology. Right now tech balance vs adversaries = CONUS leans fortress, but advances in gunpowder made fortresses irrelevant. PRC global strike with conventional ICBMs in a few energy distribution nodes and US is no better than Saudi refineries vs Yemen - it doesn't matter if US have resource autarky if you cannot protect vunerable extractive infra, which has knock on effects to everything, including (fertilizer for) modern agriculture. Even JP pierced CONUS with fugo ballons in WW2, of course it was ineffectual, but modern guided munitions are likely to be. Ask how much US preeminance is built on CONUS serenity, and what happens if that is threatened, like US can threaten everyone else with relative impunity. How many students can US brain drain in shooting war with mutual homeland vunerability, what happens to US global hegemonic framework when Boeing plants, SaaS/payment data centres, tech campuses, F35 production lines can be distrupted. Like Trump doing fine dismantling a lot of that without PRC help, but postwar US order/strength is built on the fact that US can hit others while homefront sustains the expeditionary hitting. Once homefront gets disrupted, that model stops working.
Zeihan focuses on the geo and forgets the politics, or conveniently uses biased politics for geopolitical analysis to sell chicken dinners to US supremacists. Granted a lot of PRC military / tech developements happened after his books, but anyone with half a brain cell can project 20-30 years and realize the conditions he builds off was unlikely to be the balance in the future. PRC political advantages negate a lot of geographic disadvantages... meanwhile PRC geography actually pretty favourable... there's a reason it sustained largest civilization for 1000s of years. Also reminder PRC actually per capita calorically self sufficient, use to be net fertilizer export via coal gassification (which PRC has unlimited in reserve, and only scaled back due to enviroment), increasingly energy independant via domestic renewables... and declining net population will only reinforce their domestic autarky - which will still remain vunerable to US strikes, but the equation changes when there is mutual homeland vunerability. Then equation shifts to attrition... who can hit more, who can endure more... and let's just say PRC with 4x more population, has 4x infra to degrade. Maybe currently US can project >4x fires at PRC mainland... but maybe in 5/10 years maybe not. Maybe PRC A2D2 enough to dismantle most of US security architecture in region, and US hedge on exquisition expensive munition delivery platforms (carriers, b21s) is less survivable than PRC brrrrting conventional hypersonics that can hit CONUS from 1000s of survivable land based fortifications. There is a lot of uncertainty in the tech stack. This not mentioning US MIC isn't calibrated for adversary size of developed PRC. Took 5 carrier groups + regional basing and multiple weeks of not sustainable high tempo operations to dismantle Iraq that's (generously) 1/100th PRC in industrial capability, all of which is deployed in PRC theatre and explicitly designed to counter US MIC.
TLDR is Zeihan makes many invalid points, some points that maybe valid 10-15 years ago, but wouldn't have really stood test of time, even then.