GDP is subtle broken in many ways
A "in general" correlates with geopolitical power doesn't necessary mean you can point at a specific case look at their GDP and come to a good conclusion. Especially not if it's as much of a special case as China.
> wouldn't that imply India, and then later Africa are going to be superpowers?
which might actually happen. If the US falls further from grace and the situations stays like that for a few decades, or the US and another huge power (e.g. China) fall into some costly conflict then India or a union of African countries have a serious chance to become a world super power.
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=> But more relevantly this discussion is going into red hering argument direction:
- No one argued that you should go _only_ by population, the argument is, that if a country with a gigantic population shrinks hugely they still have enough people left to wage large scale wars.
- Similar no one (should) argue that GDP is by itself meaningful for such an assessment, it's one imperfect not supper reliable statistical point. Especially given how things move wrt. dark factories (i.e. fully automated => no light one as no human there), AI and similar. Yes, if GDP is supper low or high that says a lot, but a lot is not everything and a average GDP for decade long trends in a word looking to face a major time of change means not much in any direction.
- Similar we are speaking about decade long developments, current military strength ist also just another non fully reliable indicator. E.g. Germany post WW1 had close to actual 0/no military strength, and then there was WW2 just a few years later...