By the time Pakistan starts prepping it's nukes everywhere from New Delhi to Washington will know and there will be incredible pressure and even threat of a pre-emptive non-nuclear disabling attack.
If the Pakistani's keep going on their path despite all the warnings - India, Russia, Iran, US even China would intervene militarily with pre-emptive conventional strikes using missiles, alternate weapons and special operations teams.
If beyond all this incredible opposition, the Pakistani's do manage to launch a nuke against India - by the time the nuke enters the trajectory and starts the ballistic approach - India would have launched a full scale retaliation against Pakistan as per Indian nuclear doctrine which is simply put : "No First Use of nuclear weapons; any nuclear attack no matter it's magnitude will be met with full retaliation".
Then there would be a mad scramble to disable all Pakistani nukes which are in flight - our new anti-ballistic defense systems are tested but never used in a real nuclear situation (thankfully for everyone)
This would be while US and others would be trying to pressure India as well to stop any retaliation against Pakistan despite them nuking us. It would fall on deaf ears though. Since any government that refuses to retaliate against a nuclear strike on it's civilians would be overthrown and reviled for centuries by the people.
END RESULT: The entire conflict lasts atmost 6 hours with Pakistan becoming a desolate wasteland while India has is more or less intact due to larger area/population (worst case scenario - most of their missiles are launched and they get through the defensive shield leading to loss of upto 700 million people).
There is a massive loss of life like never seen before in History (easily over 200 million since Pakistan has about 180 million people and India would lose atleast 1 or 2 cities to the nearly 200 Pakistani nukes) with catastrophic ecological damage, radioactive fallout, world weather pattern changes due to all the particulates launched into the air, partial world winter might happen, decrease in sunlight and in air quality and worse - NOT TO MENTION the people would be suffering ill-effects from all of these - radiation poisoning/burns/etc, genetic disorders, reproductive disorders/babies born with genetic defects, rise in harmful mutations, etc.
India's economy would take a massive hit due to the loss of life and habitable land and potential political scapegoating that we would be subjected to while we would need to re-organize the Government, the people, the economy and the military while trying to get aid for rebuilding.
Then we would have to find a way to terraform all the affected lands back to normal - which will take who knows how long.
Though on the extremely minor positive side - there would be no more Pakistani based terrorism in the world and India would acquire that irradiated territory which would provide direct access between South, South East, Central and East Asia not to mention to Russia and Europe too which could alleviate our power supply problems though at that point - what would it matter?.
This is why I hope no country ever uses a WMD again. The results and the potential cascade scenarios : the Indo-Pak nuclear exchange goes Global similar to how WWI escalated and the current Human civilization with most of our population, our fellow tenants (flora and fauna) and our poor Mother Earth would be dead. Armageddon/Ragnarok over.
The survivors might survive, rebuild and a few thousand years later when excavating our remains will discuss on how stupid we were.
Update : To commenters here are some links to sources :
The Day A Nuclear Conflict Was Averted
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