I agree there are many other factors in play, but the question is how do those factors compare with population? We can answer this objectively, or at least as objectively as equating GDP with economy, allows us to do. This [1] is a list of countries by their GDP per capita. It's quite surprising how closely all countries tend to fall together. The difference between Greece and the US in terms of economic output per person is less than a factor of 4.

With China having well over 4x the population of the US, this sets a more visual benchmark: should China achieve the aggregate output/capita of Greece, they will have overcome the US economy. Of course the catch is we're speaking in nominal terms. Looking at something like the GDP per capita, in PPP terms [2], can help indicate how difficult a journey this will entail. A country like South Korea has a low GDP but is likely hitting closer to the tail end of their rapid growth phase. In spite of having a very low nominal GDP, they have a very strong domestic economy with an output/capita rivaling the West, in terms of local spending power. There is less low hanging fruit, for them to pick, than their GDP would suggest.

But China is in a different bucket. Their economic output remains low per capita in both PPP and nominal terms. They have substantial room for rapid upward growth and continue to go in that direction. They will certainly overcome the US, and in the longer term picture they will likely end up with a larger economy than the US and EU combined. I'm anything but a cheerleader for this outcome, but I don't see any realistic scenario where this isn't the case.

[1] - https://en.wikipedia.org/wiki/List_of_countries_by_GDP_(nomi...

[2] - https://en.wikipedia.org/wiki/List_of_countries_by_GDP_(PPP)...