The idea of being able to "destroy" a nation, without fear of retaliation, seems somewhat silly when you consider nuclear armed submarines, nuclear weapons based in other countries, etc.
Besides, a submarine doesn't have to launch within minutes, the captain could decide to launch days later after weighing his options and discussing with the XO and other senior officers on the submarine how to proceed given that the high command was wiped out...
They probably have contingencies for such possibilities.
If I were the captain of a nuclear armed submarine and learned that 24 hours ago the nation had been devastated by a sneak nuclear attack and that the executive was gone, most of the legislature was gone, the joint chiefs were gone, and most of the strategic command was gone, I'd take it on my own initiative to conduct an immediate retaliatory strike against the offending nation's primary population/industrial centers.
Unless an aggressor can neutralize every single nuclear missile armed submarine they will have to accept the loss of at least a half-dozen or more of their major population centers.