Unskilled labor is easily automated...
For example, one of my jobs is a professional translator. Becoming a good quality translator requires at least 5 years of pretty intense study in order to become fluent at the language, and probably another 3 or 4 of reading different subjects so that you understand what you're translating. In total I would say an average of 7 years to become a good translator. Translation, in this sense, is a "high skill" job.
However, due to the advent of CAT software, or Computer Assisted Translation software, it has made much of the grunt work of translating--word lookups, phrase lookups, consistency throughout the entire translation--much easier. However, this also means a drop in the income for translators, who are usually paid on a per word basis. Now, I don't use CAT software apart from google to do my translations, but I realized that the average socially necessary labor time to complete a translation has since become pushed down by the CAT software. Even though I am certain I am probably one of the top in my language pair (I am native in both languages), and my translations turn out better than CAT assisted translators can churn out, and my language pair is quite rare (i.e. there are very few people that know both languages that I know) the average industry price is actually pushed down as a result of the CAT software.
Now, in order to reproduce a translator requires at minimum 5 years, but probably 7 years, but to replace a janitor requires maybe 1 or 2 months. Translation isn't so easily automatable in its entirety due to the complexity of the job, but janitorial skills are quite easily automatable. Yet, in the example above, it's already clear that I've experienced a pressure on my wages, even though my work is more skilled than a janitor's (I'm not knocking on janitors, I have been a janitor and have family members who are janitors, but it's just a fact that my work now requires more skill), despite people with my skills being quite rare, and there being a demand for my work. I mean, I get consistent work all the time, so there's people who always need translations. But the wages are still pushed down, despite the relative lack of competition.
So, to summarize my point, it's not as simple as supply of labor going up or down that explains how wages work in capitalism.