"Since the enactment of the current Presidential Succession Act in 1947, the president pro tempore is third in the line of succession to the presidency, after the vice president and the speaker of the House of Representatives and ahead of the secretary of state."
That’s not how the system works.
How does it work?
Dinosaur Grassley would become president.
"Since the enactment of the current Presidential Succession Act in 1947, the president pro tempore is third in the line of succession to the presidency, after the vice president and the speaker of the House of Representatives and ahead of the secretary of state."
https://en.wikipedia.org/wiki/President_pro_tempore_of_the_United_States_Senate