It's a trendline. For much of history, it's been cheaper to retrain a displaced (by technology) worker into a new position created out of the surplus wealth. But this rule of thumb needn't be true. The ability to create a robot to do the new job should speed up over time while the ability to train the worker into the new position will not.
But some of the wealth inequality is due to technology. When I was younger, the company bought the computer and hired people who knew how to use it. They captured most of the gains from my productivity because they paid for the capital investment and my education was easily transmitted to others (I didn't have a competitive edge in using a commonly available piece of software). Nowadays, the company doesn't even need to buy the computer. Many jobs require a scare capital to charge a premium for their services. Sometimes it's physical (like, I need a cement truck if I want to get paid big bucks helping deliver cement to a construction site). Sometimes it's artificial (I need a ticket from the plumbing association to get hired to plumb a jobsite). In both cases, a robot could (eventually) replace me, and it will be nearly impossible to retrain me to do something that would earn me a greater wage (or I'd be doing it already).