I disagree. Every Olympics there is a swimmer who is able to dominate a wide range of distances and strokes-- Mark Spitz, Matt Biondi, Janet Evans, etc. Look at Missy Franklin this year. It's just not rare to see a swimmer with a huge range of events. What is rare is to see them extend that over three Olympics.
That's why I argue for Carl Lewis. The 100 meter/long jump double just isn't done in modern track and field. It's basically two different sports-- much closer to being a great swimmer and a great diver than being good at the crawl and the butterfly. Lewis won gold in both twice, added a 200 meter gold in 1984, was on a couple of winning relay teams, and won four straight Olympic long jumps. I honestly could see another Phelps emerging someday only because it seems there is always a dominant swimmer in each Olympics. We'll never see another Carl Lewis.