Again, you need to explain why the Speaker of the House is necessarily disqualified. Congress works with the President. Congress was the one that confirmed a good portion of the Cabinet.

In an emergency, shouldn't the person who has been doing the most to actually run the country (Congress) be the one to step in rather than one who only deals with a single facet? Do you really think someone who was chosen to be Secretary of Defense is going to be a good person to handle the economy, the interior, the environment, health, education, welfare, etc.? Compared to someone who has been passing all the legislation regarding those things?

Wouldn't the person who passed the legislation be more capable of enforcing it?