right its economics but then whats your point suggesting silo over ssbn ?
SSBN has many drawbacks:
1. Higher service rate: SILOs can be hidden permanently & with radars to detect incoming missiles, any SILO which is deep in India like Vindhyas, Western Ghats, Eastern Ghats, Aravallis, Odisha, Chattisgarh mountains etc can be fired as soon as incoming missiles are detected to be incoming, preventing any first strikes, even if SILOs are somehow spotted. Submarines always have downtime of 30% as there are supply needs like food, sanitary equipments & fatigue issues
2. Cost: SILOs cost much lesser (1%) per ICBM whereas the submarine cost per ICBM (cost per submarine/number of ICBM) is way too high. Remember, each missile in a SILO can carry MIRV too. So, if there are 500 SILOs, there can be 2000-2500 warheads. Also, since ICBM needs to be placed in both submarine & SILO, only the cost of container is in question, not cost of missile. This means we must only compare cost of making an underground structure containing the missile vs submarine & exclude missile cost itself. So, cost of a SILO becomes 10 crores (mostly concrete & launch mechanisms) compared to 6000 crore for S5 submarines with 6 ICBMs or 1000 crore per ICBM!
3. Bad communication: Since it is underwater most of the time, it can't get quick & direct communications. Even deploying buoys to get indirect communication can be detected by SIGINT. Hence even in case os wars, the time to relay the info to SSBN will be too much and hence its attacks not very effective
4. Missile preservation: Being moved around in submarine, TEL etc takes a toll of missile life. But in SILO, it can be preserved for long time by maintaining air condition and undisturbed environment.