Islamic extremists are not the only security threat to the USA. We continue to exist in a relativel tense peace with China, Russia, and other nation-states.

The ability to drop a bunch of nukes on someone on short notice is not to deter terrorists, it's to deter nation states - anywhere between North Korea through Pakistan.

The existence of nukes and the submarines that carry them is almost entirely orthogonal to the (relatively new) thread of terrorism.