Imagine instead of people we're talking about horses. Horses have had a variety of jobs throughout history. They bounced around between farm, military, and transportation jobs as different trends and technologies came and went. Horses didn't have to worry, there was always something they'd be useful for.
And then, within a 50 year span, they lost almost all those jobs, because machines surpassed them in their core competency (pulling and carrying stuff).
Similarly, humans will get bumped and jostled around and generally will have something to contribute... until we quite suddenly (from a history perspective) don't. A few more advances in artificial intelligence and robotics, and the majority of humans will have nothing to contribute to the economy. The kinds of jobs that humans will still excel at (eg. creative stuff like writing) are also things that just don't require that many people to do, and which many people will continue having no aptitude for.
This is good news. It'll be awesome to see what humans can do post-scarcity. But the transition will be awkward.