Nuclear deterrence is more about perception than numbers, and as long as the other side perceives a survivable nuclear capability, deterrence will hold. Indian nuclear weapons are primarily against China. They are the first one to introduce the no-first-use(NFU) policy. Why is that? Do you think they are a peace-loving nation? No. But to reduce the risks of nuclear war (or any direct conflict for that matter) by giving a high degree of deterrence against the adversary. CCP understood the role of nukes to keep the peace (ironically).
China was threatened by USA of nuclear strike in Korea war and hence China decided to go nuclear. China is not aiming at India nor has it threatened India with any nuclear strikes. China is not a bunch of jihadis
The NFU depends on the fact that If any of the nuclear stockpiles survives the 'first strike' then it will become a
mutually assured destruction (MAD) scenario. In the case of india-pak, even a single strike on indian soil will annihilate the pakistani state as we know it (But we might survive). Thus, We are assuming 'first strike' won't happen because the other persion is not that stupid.
The NFU is just a gimmick. The missile launch can be detected by radar and retaliation can be immediate. There is o need to wait and watch whether the missile in a nuke or not.
By the way, nuclear bombs will not destroy everything, not eve complete Pakistan. Complete genocide will require massive ground level action, aerial bombing, logistical warfare etc. Nuclear bombs are big bombs but not all powerful taht entire countries can be destroyed. 500 nuclear tests were made in USA and not even 1 person died because of tehse OPEN ATMOSPHERE tests. It is bogus that MAD is caused by nukes.
MAD is caused due to large quantity of production available due to industrialisation. For example, India makes 50 lakh cars and commercial vehicles annually. This level of production when channeled to military, will produce massive quantity of equipment and can be used to wage MAD war. Nukes alone are not deciding here. Otherwise, there would be no reason to make planes, helicopters etc. Just throw nukes and war ends.
Your above post says land launched ballistic missiles can be anywhere hidden. Then what is the point of 'First strike'? We will take heavy casualties when they strike back. afterward, even if we win, at least a third of the population will be agony. It will take a lifetime for the nation to get back to a state anything compared to where we were. This is not a video game to send 'thousands of ballistic missiles' and expect to be safe.
The incoming missile can be detected at least 10 minutes before they come as launch ca be detected. It is not true that 40 crore will be hit just by nukes of Pakistan. One single nuke of 200kt used as warhead can destroy about 4-5sqkm. Even cities like Delhi has 12000 people per sqkm. So, 5sq km will be just 60000 people. Think again before exaggerating the strength of nukes. The nukes are only the first wave. The war if started will take years to end. Mere missile strike is insufficient.
There is a perception that with new space-based sensors and air domination it is possible to neutralize all land-based TELs (Anti-ballistic missiles taking care of the few missed ones). Submarine based deterrence is to give full confidence to the adversary that there will be missiles lefts in the oceans even after we get annihilated. The rapid weakening of the balance of power in the subcontinent, explains the eagerness of Pakistan to demonstrate their sub launch capability.
Pakista does ot have submarine missiles but India does. Also, as I said, missiles can be detected at launch and there is at least 10 minutes time to fire back instead waiting to be destroyed. Essentially, entire concept of surprise first strike is null and void due to radars. This is true for both India ad Pakistan
Global perception is another part, There no moral reasoning to do a nuclear first strike. Even after if we survive, How will you justify killing millions and starting a nuclear war (only the second time in history) ? How will you justify polluting the whole earth? (Every human will bear the cost aren't they?)
Earth will not be polluted as seen in 1000 tests of USA- Russia, - UK, - France - China etc done in atmosphere. Also, killing people who want to kill you, even if it is in billions is justified. There is no need to show special consideration to international community. The international community is nothing but the western whining countries. Killing enemies is always justified except to those who are on the enemy side.
Now, Arihant is a pure SSBN design evident from the hump and the choice of the reactor. Tonnage is not the differentiator of the two types. We chose to develop SSBN first because we felt the need for credible deterrence more than the urge to rule the Indian ocean (obviously!).
Hump means that there is space for missile. But how does it mean that it can't be used to hunt enemy ships and submarines too? The reactor is Akula submarine is also PWR reactor ad there is nothing special about Indian reactor that prevents it from being used for attack. It can have dual role. Ruling Indian ocean is more important that getting few missiles in the sea as I said above about the futility of first strike. Why first strike is useless:
1) The launch can be detected by radars. The missile can also be detected while flying in the sky by radars
2) The missiles are stored in silos underground or in TEL which is mobile
3) Impossible for any strike to penetrate 10 feet underground and take out the silos as the enemy missile is not having penetration warhead.
4) The radar detecting a missile flying towards India will give the ballistic trajectory and eve tel where the missile will land which means the silos in that area can be asked to launch immediately before the first strike hits
5) Missile silos hidden under mountains is impossible to be hit by first strike. Since the number and area of such mountains are large in India, India ca store thousands of missiles under mountains whereas the 20-30 missiles of submarines will mean very little.