You also have to look at current American Marathoners times vs. the current world record. Roger's 1979 Boston was only ~20 seconds off the world record. Salazar also ran less than a minute off the world record at Boston, New York, and Fukuoka and arguably his best marathon in NYC in 1981 would have likely been within 5 to 10 seconds of the world record had the course not been short.
At Boston yesterday and at most marathons as of late, the top American finisher is usually several minutes between the winner and 5 to 7 minutes off the world record. It really does show how much American Marathoning has declined in the past 35 years. What's odd is it isn't like we don't have great marque races, like Boston, NYC, and Chicago and training and shoes have come very far since the 1980s.