Presently, neither India, nor Pakistan possess an effective incoming missile identification and Ballistic Missile Defence capability, irrespective of Indian claims (a couple of Israeli radars wont make much difference the one system which DRDO has opened to reverse engineer, I hear they dont know how to re-assemble it).
Even the American BMD capability is rather limited and does not provide an assured hit probability for all kinds of missiles, particularly those with anti-BMD defence capability. Which means that both India and Pakistan can hit each other with nuclear ballistic missiles with impunity from anywhere, even from the subs, nuclear or not.
This also indicates that the Indian nuclear sub can fire this missile from Bombay harbor or from the Himalayas in the north or from Shangri-La itself, Pakistan cannot stop it. Pakistan can do the same as its missiles cover the whole of India and can devastate India and India can pretty much do nothing about it, irrespective of where the return second strike is going to come from. With current capability, Pakistan can and would respond to destroy over 100 Indian cities in the initial retaliatory strike. India can retaliate with the secondary strike from anywhere. For a credible minimum deterrence against India, Pakistan should be able to destroy a minimum of 400 Indian cities in the first strike.
Therefore, despite this SS Something N, India and Pakistan can destroy each other irrespective of this machine. The importance of Indian hired Russian sub is more for Pakistan than this tinhorn. Arihant is dangerous for China and for those countries where, currently Indian missiles cannot reach or may not reach in future. The third triad for assured second strike capability, whenever it is operational in mid to long term timeframe may only create difficulty for China. It does not matter for Pakistan at this juncture.
Let me also highlight a bit of BMD agony. The solid fuel Shahab-3 takes about 70 odd seconds to consume its booster motor. It currently, takes about 15-20 seconds to figure out that a missile has been launched from the whole swath of Iranian landmass through either satellites or land-based radars. The system also takes some time to identify if the missile is nuclear tipped or not and then, passes this information to the ground stations, taking another about 10-15 seconds. Time is needed by the counter missile battery to identify and acquire the target and launch the counter missile, which could be another 10-15 seconds or more. Once launched, the missile may just have about 15-25 seconds to neutralize the target missile. The Americans are launching the faster Block-II missiles to ensure that the counter missile can hit the target before the booster motor extinguishes. This is just to explain the boost phase defence. If the missile is missed, then comes the intermediate phase defence and finally the terminal defence, where Arrow-2 etc come in. Impossible no. Difficult yes extremely difficult. Does the Indians have this kind of integrated technology not at the moment. And please, dont start day dreaming, itll take you a long time if at all to reach the stage where you have a complete system.
You guys may say that India will have an effective BMD in future. By that time, we would also have something similar and would also have improved our strike systems to avoid Indian defence capability.