Skill is a proxy for supply whereas pay is a proxy of supply-demand. It might actually be a better term, since even if you do very skilled labor, you would prefer an union if there was no demand.
This is a terrible substitute, since there are a lot of low-paid high-skill jobs (ex, graduate students, TAs).
There's a meaningful labor liquidity difference between a job that takes 2 years of training and 2 days of training, and it's important for policy decisions. Sorry?