That is the one of the key fallacies in any of these assumptions. What is to continue of India if every major city and military target is hit?You do not have a ABM. And S-400 is NOT a missile shield at ALL. It can perhaps intercept scud but thats its limit. S-400 is excellent air defense system but its NOT a ABM.
Lastly, if you want to go "all out", why do you think your opponent cannot start a war with "all out" nuclear option as well. Remember, you do not have a SSBN. At best a SSGN. If they go all out for decapitation, its all over for you.
Nuclear war will be end of Pakistan and devastation but still continuation of India.
What is going to continue it?