Has China lost then? The USSR broke up after 1990. If one nuclear armed country breaks up, why would a neighbour lose?
1. The original argument in this thread is whether India will lose, not whether China will.
2. On that argument, if Pakistan loses disintegrates, it is extremely probable that Islamist elements will get their hands on nuclear devices.
3. If Pakistani Islamist elements get their hands on nuclear devices, India will be their first target. (Yes, I know China, the US, and Russia would also be possible targets for said elements, but India would be the most convenient and most hated target).
4. Ergo, if Pakistan disintegrates, it is likely that India would suffer nuclear terrorism.
5. If India suffers nuclear terrorism while Pakistan does not exist as a state entity, there is no legal target for Indian nuclear retaliation. (e.g. If India retaliates by nuking a Pakistani city when it is clear Pakistan's government did not nuke India since Pakistan's government has ceased to exist, that would be an illegal act, and the GoI is complicit in a massive war crime. Under international law, civilians of a specific area are held as innocent of the actions of a non-governmental entity that happens to also reside in the area.)
6. Ergo, if Pakistan disintegrates, it is likely that India would be placed in a situation where it had to suffer nuclear attacks without being able to respond in kind. The only solution would be occupying Pakistan with a mix of infantry, spec ops, and drones to hunt down terrorist figures.
7. Certainly, the IA would be justified in occupying Pakistan to root out terrorist elements - but that would make the Soviet occupation of Afghanistan look like a brawl between primary school students; the casualties would be horrific; since COIN is messy, the IA would inevitably be complicit in war crimes.
8. If the IA occupies Pakistan, the constant drain of manpower and materiel for pacification and rebuilding parts of Pakistan would impose long-term economic costs on India at a time when the Indian economy would already be weakened by the specter of nuclear attack.
In fact China, Pakistan's "taller than the tallest mountain, deeper than the deepest ocean, sweeter than honey" all weather friend stands to lose more by Pakistan's break up than anyone else. Pakistan's degeneration after Chinas robust support to Pakistan against India is one of China's worst politico-military defeats. You can still turn it to victory by supporting and stabilizing Pakistan.
9. Returning to your argument, the occupation of Pakistan by the IA would also weaken India's strategic position vis a vis China, by tying down large parts of the Indian army in COIN campaigns in a deeply hostile land as well as diverting large chunks of the budget from high-end platforms to fight a conventional war to low-end platforms designed to counter an insurgency.
10. It would also trigger a response by the entire Muslim world against India. Saudi Arabia, in particular, would be enticed to support an insurgency campaign against any IA occupation. Think Syria, but worse.
11. The Western powers, Russia, and China would all thank India for shouldering the noble burden of killing so many terrorists, and go on their merry way of making each other rich while India loses a decade of growth, perhaps more.
In summary, a disintegrated Pakistan creates immense strategic issues for India. I am curious to hear what benefits for India you think could exist, and why they would outweigh such costs.