A simple math: How much this S-400 system costs? Training, support systems, deployment and maintaince costs?
How much simple unguided rocket systems costs? How much training is required?
Excluding artillery long range effects, what is the cost benefit ratio for the Indians to deploy the strategic assets in ubber hostile enviromnets. Is it possible that defending forces would have already war gamed scenarios of asmysteric defence and denial strategies.
IMHO there is no rational logic in deployment of this system anywhere near 400km zone. Might be intended for strategic defence of seat of government and other strategic assets deep within India from Pak BM and CM missiles.
Hopefully, war never happens in this region. Only way India will ever break out of South Asian box is by creating permanent peace with Pakistan. It will require rational decision. However, IMHO, in the current situation stability is not going to happen in the next ten years. It is sad.