There’s also nothing to say that a submarine has to actually be underway during a deterrent patrol. A while back, photos of the marine growth on the hull of a certain non-Russian, Non-America, Non-Chinese, Non-French, Non-Indian, third party SSBN proved that it had been sitting stationary, underwater, for months on end during a deterrent patrol. It goes to show that established nuclear powers can use SSBNs as stationary missile launching barges. The high specific heat of water means that a stationary SSBN on the bottom in a protected “bastion” safe from enemy SSNs is a lot less vulnerable than any concrete ICBM silo.