No one knows what will happen in the future. We know America is demographically collapsing and changing its identity from white to black-hispanic
We know China is 96% Han or Han like ethnic groups (not including Tibetans/Uighurs) and survived 5000years and keeps its DNA in tact so it will obviously survive much longer
India is also keeping its DNA and demographics in tact but it doesn't have the ethnic/racial unity that China has since India was more of a continent then a country throughout history. Most Indians identify as their caste, state, territory then an 'Indian' identity but there is nothing wrong with that.
Fact of the matter is India can do well if they cut the rich/poor gap and get those hundreds of millions out of slums. A big young population is the biggest disaster if you can't find them jobs and give them hope. They turn into criminals, looters, drug dealers, prey on their own people, don't pay taxes and cost the government a lot of money (like african american population in Detroit/Chicago/Memphis/Baltimore etc). Not to mention the difficulties with overpopulation causing rising rent prices, lack of jobs and opportunities and as a result creating civil disorder/disobedience.