Prior to the change from the original Delta Frequent Flyer program to Skymiles (on May 1, 1997, I believe), the program was structured in such a way that you could earn a minimum of 2500 miles per segment. A first class domestic ticket was 25,000 miles and coach domestic was 20,000 miles. So essentially, it was 8 segments for a coach ticket (just like WN), and 10 for a FC ticket.
Now, I just booked 3 domestic coach tickets, @ 52,500 miles each, and I was lucky to find the seats... At an average of, say, 1200 miles earned per revenue segment today, that's 43 flight segments for a domestic coach ticket, if you can find it.
43 segments/8 segments = over 5 times as much flying required for a single coach seat. Even if one can somehow find a seat that you can actually book for 25,000 miles, that's over 20 segments for one free domestic ticket.
Ah, memories...