The thesis of the permanent underclass has two core knock on effects that are largely under allocated.
A free market economy is largely a competence hierarchy composed of individuals and their ability to generate value for others. Think of it as each human having a score that is some combination of ambition (hours worked) x genetic predisposition (ie IQ) x leverage (technology or resources).
While neither IQ nor ambition scale, technology does. A 100 IQ working 30 hours will overtime beat a 110 IQ working 40 hours given that the 100 IQ uses technology in a way to either double his output, or make him produce the results of a 120 IQ.
Now naturally in a free market economy, this experiment will lead to top percentile IQs that work every waking hour, trying to outcompete each other.
The permanent underclass is the population that falls below the line where neither them without AI, nor them with AI can compete with a standalone agent. As of today, this line does not really exist yet, as virtually all AI products, still require a human manager.
Thus the North Star for both those wishing to escape that ever rising base line and those wishing to compete at the very top will try to find ways to increase their score.
Where can people get advantages from?
Enhancing their own compute (brain power) both technologically (BCI/AR) and organically (biotech). Equally we may see physical power enhancements (mechanically and biologically), where the cost of enhancement is a fraction of a comparable humanoid.
Now assuming all the above is true, two knock on effects become unavoidable. Enhancing yourself will be a core tenant of the upper class, while finding experiences to fill the void left by exiting the productive class will be a core tenant of the underclass.