"In the grand scheme of things overpopulation isn't that big of a problem"

Depends if you want to be wealthy. Overpopulation limits the wealth potential of the people that live there. China having less people than India is a good thing.

A collapsing population is a bad thing. A stable not growing one for a large population country is good.

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The problem in Asia is the youth population 0-5 years old is almost half the 40-45. A stable population would now require 4 kids per family to avert losing half the population. That is not good.

I wouldn't be surprised if the population gets even more lopsided in India's favor when many of the single Chinese men learn there are plenty of young women in India and immigrate there. China has a huge imbalance in the male:female ratio.

Sorry but data disagrees with you. A lot of experts are saying China is heading towards what's happening in Japan. And we already have data for a stable population rate. China is below that. China is in trouble, not India. India's population will stabilize. Of course climate change threatens everyone.

To be fair though the welfare standard in Japan is still light years ahead as of that of basically the rest of Asia. We also need to grow to a world that consumes less and is more sustainable instead of only pursuing economic growth. That is if we want to live more than 250 years on this planet. For instance 2.5 degrees global warming is destructive enough already. If we hit 5 degrees that would mean the end of many civilizations.

As for threats for India, there are 2 massive threats for India one is climate change as I explained in my post on page 1. But don't rule out pollution. Indian ground may be very fertile now, but don't rule out the effect of chemicals and pollution. As a Dutch person (a country which has a very industrial approach to agriculture) I can tell you that polluting the soil and ecosystem with pesticides and other chemicals too much can drastically lower the fertility of the soil. This will lead too massive decrease in the quality of ground and surface water., to the point water and food becomes somewhat poisonous.

In the long run pollution can drastically decrease the amount food production. Leading to either hunger or a massive inflation of food prices and hunger as a result. This can all happen within our brief time on earth on this planet. So I would say India might be in bigger problems than China. Especially since China can also become the energy powerhouse of the region and has a huge amount of rare minerals/metal which is combined with energy the oil of the future.

India and China (as well as the world in general) are too populous. But admittedly, it's the middle and upper class of countries that tend to do the most polluting. I almost surely use more electricity, paper, oil, etc. than several people combined in most of the world.
It's also not good in the sense that India is smaller in land area than China and is on average more crowded.
India is only partly free, though admittedly that is much better than the highly authoritarian government of China. I fear as the India population grows; more freedom may be lost in order to maintain a grip on the massive population.