A country’s population has nothing to do with determining whether it is rich or not. It is definitely one of the inputs, but not the determining factor.
Consider Japan. Japan is a almost as densely populated as India is. Still it is richer than India is (or was until very recently) . It ultimately depends on factors like how efficient a country’s production is, how well off its citizens are in social and health indicators, how resource rich it is, how modernised it economy is etc.
Many people make the mistake of assuming that if the population comes down, then per capita income would automatically rise. However you have to understand that the country’s production also would come down if we assume a deduction in population. India’s current GDP has been achieved with the current population as one of the inputs that goes into producing that much output. You take that out and our GDP also will take a hit.
So, if the massive population is brought down, it need not necessarily result in India becoming a rich country.