1. Greeks were once the greatest nation, and where are they now?You're clearly not acquainted with history or current events. First, advanced civilizations in the Middle East, North Africa, and India long predates the earliest civilizations in what is now China by 1200-3000 years.
Second, there are plenty of intelligent Indian STEM folks around the world in math, engineering, IT, etc. Doctors of Indian descent are common in the West if you live in a developed Western nation. Intelligence is clearly not an issue.
Third, 40 years ago, China's GDP per capita was as low as the poorest SubSaharan country and the lack of toilets meant people were sh1tting in the fields and using it to fertilize crops. The Chinese Communist Party's retarded state socialist policies killed tens of millions (including policies such as mass killing sparrows/birds because they ate grains), and intellectuals and anyone with an education were considered "capitalist roadies" and were attacked, tortured, and imprisoned during the Cultural Revolution.
What do you say about a society that viewed intelligence and education as bad and had so many retarded policies? Bad DNA?
2. Romans were once the greatest nation, where are they today?
3. Mongolians had the biggest empire of all times. Nothing left from that empire.
It doesn't mean if something was great in ancient times that it's still good today.