I remember Frank Shorter commenting on this right after he won in Munich. Frank said something like, "Looking at a gold medal as being worth X amount in your future is an excellent formula for ending up disappointed." Then he said he came home from the Olympics, went back to (I think) law school, and went back to training. In other words, he went back to what he was doing--picked up from where he left off when he traveled to the Games. He's done pretty well for himself, with most of it (based on what the general public can appreciate about him) based on his Olympic success.
Bruce Jenner did alright. He was good looking and he had a beautiful wife and he won a glamorous event. However, this thinking indicates that perhaps one gets into the Olympics and trains for all those years merely to cash in on your success. It does not account for the fact that an athlete does a sport, like Ryan Hall has said, simply "because I just love to run--that's why I do this."
Gymnast Kim Zemeskel (sp?), who had a terrible Olympics after being a media darling, said as much. "I never looked at the Olympics as a big negative experience. Yes, I performed poorly, but I don't regret any of it because I did it for the love of my sport. I don't remember any of it being a sacrifice. I spent some wonderful years and I have great memories of my times leading up to the Olympics. I loved my fellow competitors and my coaches and pretty much everything about gymnastics." And on the same hand as that, Shannon Miller stepped into the spotlight when Zemeskel faltered, but it didn't exactly become a cash cow for her like it did for Bruce Jenner or Summer Sanders or maybe some others (Eric Heiden? Bob Mathias?).
Mark Spitz is a pretty good example of someone who had a spectacular Olympics, was extremely popular, but then failed at nearly every attempt to strike it rich off of his success and popularity.