Also don't forget that early in the program _everything_ counted, including award travel, partner miles, AMEX rewards and class-of-service bonus. DL didn't start counting only base miles for MM status until one of the program "enhancements" in the late 90's.

If that 4-MM started flying in earnest in the 80's then the 4MM figure was more easily attained. Even with the current AMEX program, MM status is _much_ harder these days.