The problem is that your entire argument is that
1) A large population by itself is not sufficient to be a great power
I argue this is true yes, but it is necessary. Proportionately a small population will only mean you can be a subordinate power.
2) India will never be able to industrialize
I would argue that you underestimate India. India is a great country with very smart people.
You say they will never be able to have manufacturing out of China and into India, but
.
India's steel output by 2018 was 2nd the world, ahead of Japan and the United States.
Between 2010-2017, India's Internet users increased from 8% of the population to 34% of the population.
Gradually the supply chain is moving, and alternative supply chains are being created. This is the choice of companies like Apple, and other foreign companies. Assembly can be moved anywhere with enough will and prodding from government policy. But what China cannot do is move advanced photolithography manufacturing to China, because China lacks the know-how to do it, no matter how much willpower or what the government investment is. Thus this is a vulnerability.
India is rising.
So it is a burden and a blessing, China's economy would be just as good with a population of 500 million? 100 million? 10 million?