There's just a handful of Ohio class subs and all of them should have been replaced by more modern and stealthier designs by now. It's conceivable that an enemy could locate most of them and hit them with nuclear torpedos or similar.
And even if most of the active subs survived it might not be sufficient to ensure MAD because they don't carry all that many SLBMs.
And even if most of the active subs survived it might not be sufficient to ensure MAD because they don't carry all that many SLBMs.