One of the things that was illustrated by Saturday night's attempted assassination was the danger of having the President, Veep, and Speaker of the House all in one location. If they were all taken out in the attack, our President would now by 92 year old Chuck Grassley, the President Pro Tempore of the Senate.
The President Pro Tempore of the Senate used to be a significant position and at one time it was next in the line of succession after the Veep. Now it is after the Speaker of the House. It has become largely an honorary position, given to the senior most member of the Senate's majority party. That means it is usually a really, really old white guy. Strom Thurmond held the position when he was 100 years old. Had the Democrats followed tradition, demented Senator Diane Feinstein would have been given that title, although to be fair, it wouldn't have been much of a change from Biden. When Obama, Biden, and Speaker John Boehner got together for a round of golf in 2011, a well placed lightning bolt would have made 87 year old Dan Inouye the POTUS.
Except for the Veep, the presidential line of succession is mandated by legislation, not the Constitution. It can be, and has been changed twice. I don't know why they don't remove the President Pro Tempore from the line of succession. While it is unlikely the line of succession would ever go down that far, Saturday night showed it is not inconceivable.