Completely false.Demographic projections 20 years out are probably subject to less uncertainty than economic projections 5 years out, yet nobody seems to think that medium-term economic planning is ludicrous.
If that were true then the UN wouldn't keep updating its "predications" every year. The proposition is false even as a matter of principle because the demographic makeup of a population is subject to intervention.The demographic characteristics of the next generation are more or less baked in according to the characteristics of the current population, and those are known.
Who is to say that ten years from now America will not have devolved into a dozen warring states with little to no influence on world affairs? America's political future - given its vastly higher polarization and teetering political system - is far more open to question than China's.Who is to say that ten years from now China will not have devolved into a dozen warring states with little to no influence on world affairs?
Your facetious question is a good introduction to a concept I hope you'll grasp: levels of uncertainty.Will the sun rise in the sky tomorrow?
The reason we know the sun will rise tomorrow is because we have well-tested physics theories about gravitation and the problem is deterministic (at least the two-body simplification). The reason the UN updates its "predictions" every year and its ranges are so ridiculously vast is that the problem it's trying to solve has far too many variables and is not deterministic from known quantities.
I think that explains the ridiculousness of comparing generation-long demographic "projections" with the state of the solar system in 24 hours, even as a joke.