Did you read my comment? I acknowledged that submarines would be the most reliable second strike option.
Those are a lot of "ifs" sure, but it's a numbers game. If deployed hypersonics were as numerous as ICBMs in the 80s, then it would be about who strikes first. If a first strike can reduce the enemy population by 90%, and reduce their second strike capability to kill only 70% of your own population, then striking first is in your best interest.
I am talking about the 21st century as a whole. Can we really be so sure that at some point MAD will cease to exist and someone pulls the trigger? I wouldn't be so complacent about it.