Western pop consensus on PRC is reliably frequently (almost deliberately) wrong that that the consensus _is_ the extraordinary proof, because these narratives are designed to be cope propaganda.
But the TLDR is it's the quality of workforce not the demographics that drives productivity and growth. JP TFR went below 2 in mid 70s, their economy has grown by 2000% relative to Yen. Same with SKR, TFR went below 2 in mid 80s, economy grew by 2500% relative to KRW... the sauce? Skilling up relative % of workforce to compete on higher end / higher value / higher paid industries. The current limitation with both these countries is they're small, relative to PRC, they've maximized human potential, reaching around 80% skilled workforce, basically the ceiling, they can no longer generate surplus enough talent to compete past limit.
PRC went >2 TFR in 90s, but they have so much people that they're still at 20% skilled workforce, the academic reforms to churn out tertiary only put in place ~10-15 years ago, their headroom is still very high. As in generating OECD+ in skilled talent combined per year. They're on track to add more STEM in the next 25 years (birth cohorts already predetermined) than US will add people (birth and immigration inclusive) - they're on track to have 2-3x more STEM than US total. Meanwhile their catchup in last 10-15 yeras was basically going from fraction of skilled talent to ~parity with US. The TLDR is PRC is in process of undergoing the GREATEST "productive" demographic divident in recorded history, and competitors are not remotely close. Will PRC reach ceiling like JP/SKR with >2 TFR, of course, but not until way past 2050s when they reach 80% highskilled workforce and can't replace at parity. And realisitally that's also a PRC who is multiple times larger than it is now (not 2000% but substantial).
If the full demographic argument is PRC will eventually demographically collapse, likely after most of us are dead, and growing multiple times current size because they generated a workforce larger than west can compete with... then that's going to make people shit bricks.
For reference entire PRC lol demographics narrative was popularized by Zeihan... who keeps forgetting the politics in geopolitics, i.e. US has most deep coasts for ports... except PRC dredges and builds the most productive ports; US has most naturally navigatable waterways, except PRC builds out / maintain their internal waterways with signifiantly more utilitzation. US has immigration... except at PRC scale, even shit TFR properly directed is more talent than US can realistically compete with. 4:2:1 pyrmaid? Yeah kind of PIA... except highest household savings rate in the world, and large regional CoL disparity means you can dump a lot of retirees in nice inland retirement cities one day where they cost fraction to upkeep but still maintain relative good QoL, better than what they grew up with. There's a lot of arbitraging opportunities. And in cases of upper quantiles, that 4:2:1 turns into wealth transfers to the 1 gen to start families.
If you double down on export capacity, and let's your hypothetical is true and China now completely dominates the global supply chain, where does that leave other emerging economies like India or Nigeria in the future that would also seek to climb up the economic ladder?
Not much space TBH AND PRC is unlikely to enable trade deficit with other producers like US/west to support export led growth. And reality is at PRC scale, "only" exporting 20% (i.e not even doubling down on exports) of GDP is enough to satisfy world demand in some sectors. IMO PRC fine with relegating lower end production (already happening) but will be just as protective as US/west on high end / high value (strategic) production.
What PRC does offer countries that want to catch up is access to cheap capital goods and cheap energy infra. Something PRC had to pay premium for while climbing ladder. It will be up to respective countries to arbituage accessible PRC capital and global demand to build up their industrial base... while opportunity lasts. Issue is we're in era where labour saving technology = more difficult to uplift via mass manufacturing employment and potential for export led growth is going to be increasingly limited as surplus importers try to reconfigure their own trade blocks.
But on the other hand 80% of the world , well 60% excluding PRC is consider middle income and poorer. So there is still substantial room to increase global consumption and accomodate new entrants, especially in non strategic / non zero sum sectors, it just won't be easy. Which is ultimately the limitation, a lot of underdeveloped countries don't have much competence in nation building and no ones going to do it for them. In strategic sectors, i.e. sectors where most countries can't support indigenously, or would take generations to build out (like having your own 200k aviation base that need 300 million population bloc to support), what PRC is going to offer is cheaper access relative to western incumbants. Hence PRC and west fighting zero sum in these sectors, with PRC trying to wrestle away western share, and west trying to protect their share. But the result should be more choice, cheaper choice.
IMO that's the cynicism behind PRC strategy, they will sell countries the tools to uplift themselves on the cheap, expecting most can't, while offering those that couldn't fallback/access to unparallely cheap goods, because PRC will simply have stupendous competitive advantage from being able to coordinate a lot of talent and a lot of robots across a lot of sectors linked by a lot of supply chains.