So I become a retail clerk. Why exactly should I expect a higher wage than other retails clerks? If the same thing happened to many people, I would actually expect retail clerk wages decreasing, because now there is a greater labor supply in the industry.

At least in the classical economic theory, the idea is that you and other displaced Java programmers are more skilled than the average retail worker before the switch to LISP, and at least part of those skills are transferrable. Probably not the Java programming, but at least the math, job awareness, and other various generalist skills that are part of higher-profile jobs.

((This falls apart in extremes because many job skills don't transfer or are even counter-useful, and employers have reasons to avoid hiring high-investment workers who have other better job offers available.))