There are ten primary missions outlined in the Department of Defense's 2012 strategy guidance. Three of those missions require the Pentagon to maintain an effective deterrent to nuclear war that, the guidance says, "can under any circumstances confront an adversary with the prospect of unacceptable damage, both to deter potential adversaries and to assure U.S. allies and other security partners that they can count on America's security commitments."
The CATO Institute's Christopher Preble and Matt Fay's recent article "To Save the Submarines, Eliminate ICBMs and Bombers" is a policy recommendation that fails to meet DOD's high standard for an effective nuclear deterrent. While they mention the submarine's inherent survivability, power and accuracy, they neglect other aspects required of an effective deterrent force. The deterrent force must be survivable, affordable, flexible, visible, available, credible and provide stability. While each element of the triad comprises many of these attributes, no one leg has them all.
Affordability is also a key component of nuclear deterrent forces. While the $60 billion dollars quoted for the new nuclear-armed submarines seems daunting, it pales in comparison to the $400 billion for the Joint Strike Fighter, according to the Government Accountability Office. Moreover, the entire nuclear deterrence force represents only around $20 billion per year, according to Deputy Defense Secretary Ash Carter. ICBMs are the least expensive leg to maintain. Bombers still have a required conventional role, so making them non-nuclear nets only minor savings. Thus, even with the addition of new submarines, the nuclear deterrent force is incredibly affordable compared to the overall defense budget of $526 billion.