Ron Howard, Richard Dreyfuss, Cindy Williams, and Mackenzie Phillips had pretty good careers after American Graffiti.
Happy Days and Laverne and Shirley wouldn't have ever been made had America Graffiti not been a hit. In fact, ABC didn't like Gary Marshall's original pilot for Happy Days, so they didn't pick up the series and just used it as an episode of Love American Style. After the movie came out, they went back and picked up the series.
I'm really not a Lucas-hater, honest. I'm a Star Wars fan and saw ROTH twice. But I have to concede the new trilogy just doesn't have the same quality as the originals.