My understanding is that the existence of the nuclear triad is entirely about maximizing the likelihood of maintaining a second strike capability in the event of a preemptive nuclear attack, thus providing mutually assured destruction even if the first strike succeeds.

Dark stuff.

That's exactly it, but it's also why submarines are probably the true deterrent - you might hit the airfields and you might get the silos but it's almost impossible to even coordinate the sort of strike you'd need to guarantee you got all the subs.