If the Americans were told this in the 1920s, then I would completely agree with them. However, if they were told this in the 1970s, then they are just ignorant.
China in the 1970s already had triple the literacy rate and double the life expectancy it has 20 years ago. It was already building satellites, submarines and nuclear weapons when it didn't even have an airforce or a navy in 1949.
So far, you have demonstrated a complete ignorance of what made China's rise possible, and failed to provide a single reason beyond population trip justify your contention.
Actually, here are some of the reasons, beside population, why India is better placed now than China had been in 1971, or even in 1981, to “rise” in the next 30-50 years.
1. india’s literacy rate in 2020 is 74%. Chinese literacy rate in 1971 is uncertain due to the chaos of the culture revolution. But officially Chinese literacy rate did not reach 74% until 1986. The annual growth in India’s literacy rate now is approximately the same as China’s had been between 1980-1990, when China’s overall literacy rate was the similar to India’s today.
2. The per capita productivity of Indian economy in 2020, measured by PPP, and adjusted for inflation is 6 times high than China’s in 1971, and about the same as China’s in 2004.
3. India’s total steel production in 2020 is 120 million tons, this is already second highest in the world. That is about 6 times China’s production in 1970, and about the same as China’s production in 1997. The compounded annual growth rates between 1997 and 2020 have similar to those of China’s over the same period.
4. India’s automobile production in 2020 was 5 million cars, about 25 times that of China’s on 1971, about the the same as China’s in 2004.
5. India’s total electricity generation in 2020 was about 2000TWh. China’s in 1971 is unclear, but China’s electricity generation also did not reach 2000TWh until 2004.
6. In 2020 8% of India’s population have 4 year university degrees. China’s figure in 1971 is unclear, but China did not reach 8% until 2011.
So broadly speaking, India is somewhere between 15 and 35 years behind China by different measures. There are more measures clustering around 15. India is not nearly as far behind as many people in China thinks.