The purpose of our nuclear forces is not warfighting, it's
deterrence. The triad is designed to present an enemy with an overwhelming numerical targeting problem and prevent any hostile technological breakthrough against any single leg of the triad (e.g. an antisubmarine warfare breakthrough) from negating the deterrent effectiveness of our entire nuclear force. To even contemplate a first-strike against us, any adversary would have to be confident in taking out all three legs during an initial strike, which is nigh impossible.
Diluting the triad's deterrence could mean the destruction of our country as we know it during an initial nuclear exchange, even if our second-strike response destroys the enemy, so the only thing more expensive than effective nuclear deterrence is the devastation of nuclear war itself. From that perspective, the triad is a bargain that has proved its value for over a half-century. Because we only get one chance at deterring a first strike against us, it is absolutely essential that we err on the side of an unassailable system of systems and a the triad has proven it provides exactly that. Alternatively, gambling on a "good enough" nuclear deterrent is potentially a recipe for disaster.
As for land based missiles in particular, they are nearly 100% ready and are cheaper than the handful of SSBNs, which are typically less than 50% operationally ready.
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