That’s why there is always at least one presidential cabinet member who does not attend these speeches, on the off chance that a decapitation strike terror attack wipes out the president vice president and members of Congress.
That was the premise behind the TV series ‘Designated Survivor’.
And a version of that was part of the Tom Clancy book ‘Debt of Honor’. Jack Ryan, having just been voted in as Vice President by acclamation of Congress, was in the lower level of the Capitol Building, on his way to the House Chamber, when the Japanese pilot flew his 747 into the Capitol. He was the only surviving person left in government and it fell to him to become President.