Most people are not above average. Meaning most employers will not have a choice but hiring below average employees, whether they're in a union or not. And most employees will benefit more from raising the floor than they risk getting disadvantaged by lowering the ceiling.

And yes, it probably does depress salaries for the people on the top of the curve. But they are also often not the highest performers either, but the people best at marketing themselves. Sometimes the two intersect, but often it does not.

The reality is that what you're suggesting in terms of employers avoiding union employees rarely is an issue in countries with high proportions of unionized employees.