I'll grant your premise, for the sake of argument.
So where's the big negative fallout? If the set of unemployable people has been growing for centuries, where is the mass starvation and misery? How can it be possible that the 20th century -- well into the process you describe -- saw a burgeoning middle class in the West, and then later a burgeoning middle class in Asia?
Clearly the benefits of productivity growth have been bigger than the overhead required to support the people who cannot contribute.
I'm not arguing that everybody is infinitely retrainable. I think the biggest problem we face is several generations of existing people with completely obsolete mindsets, who are too old to relearn.
Your whole argument is written in terms of employment at a job. But that's precisely part of the mindset that is obsolete. The idea that "getting a job" is the best option for most people is historically recent and already dying. There are plenty of other ways to organize an economy, and there's plenty of historical precedent for the idea that common people can successfully operate far more independently than they do under the industrial model. In a capital rich environment, you teach people to be capitalists, and let a million independent experiments blooms.
The alternative is you keep teaching people to defer to the boss, and the boss evolves into the Lord, and we go back to serfdom. That could happen too.
Assumptions about what "average people" are capable of need to be judged against the system that's training those people. Our present system was consciously designed to make them into good industrial widgets. But the malleability of children is absurdly high, and from directly experience I see no reason you couldn't turn nearly all of them into creative capitalists.