I think the problem is that unions are famous for fighting for equal pay across the board for the workers they represent regardless of individual competency or market demand. For this example they'll give COBOL developers a raise to 120K and give web developers a pay cut to 120K.

Or best case scenario they give the COBOL developers a short-term raise to 150, then raises across the industry stagnate in coming years to offset the fact that employers feel like they're overpaying for some people. But sure, a few years later the union can come in to look like a hero arguing for a fraction of the raise the web devs could have already gotten.