If something were to happen to the President, the vice president would assume the presidency.
If something were to happen to the President, and then to the vice President, the Speaker of the House would assume the presidency.
The younger generation of Democrats want new people in leadership positions. That's all well and good, but during these trying times, should the Speaker need to assume the presidency because both Trump and Pence go down, I want a smart, skilled person for the job. Not a greenie. I've got your back, Nancy Pelosi.