But that only applies if the one ahead of them is dead, or not eligible to be President. For example, SecState Kissinger could not have become President under the rules of Presidential succession, since he is a Naturalized Citizen.
But even Biden isn't stupid enough to put Hillary a heartbeat away from the Presidency, not when it's his heartbeat.
Hillary! is by far not the most dangerous person to have near the oval office, as events have shown. She's openly polarizing and would have a natural "gridlock" effect.
"Gridlock" is Good and was designed into our constitution. It's better referred to as "Checks and Balances", but whatever.