Nobody has mentioned Clarence DeMar 7 Boston Marathon wins his first in 1911 and his final Boston win in 1930 so 19 years apart, so if Rupp wins Boston in 2018 he would have to win in 2037, DeMar also has a 1924 Olympic Bronze Medal in 1924, I think Rupp will be a great Marathoner and will winsome Marathons (5-7 would be my guess as to how many Marathons Rupp will win in his career) I hope he runs at least 2:04.57 or faster so I don't have to explain that 2:05.38 is the American Record, even though American Ryan Hall ran 2:04.58.
Who is the best will always be debated, Shorter had Olympic Gold in 72 and Silver in 1976, and 4 wins at Fukuoka, Rodgers 4 wins at Boston and New York and Fukuoka and a lot of other wins, Salazar was superb, but a short career Khanouchi and Buddy Edelin brought the World Record to the USA, Meb ended long droughts for Americans with wins at Boston and New York and the Olympic Silver.
Rupp will be different, he will likely be much faster than any American, It will be exciting, he has not run fast yet, but you know he will eventually.