Back in the day, there was little/no protection for employees and unions were needed to provide even the most basic safety standards within companies ...
Today large Unions are effectively a monopoly on labour and eliminate the ability of the company to control the quantity, quality or cost of work provided by its employees; and large Unions (in particular public sector Unions) tend not to look out for the best interests of their Members, and are only looking out for the (often political) interests of the Union leadership.
If you created a limit on the number of members to a Union (1,000 in a multi-company union, 2,500 in a single company union), you disallowed multi-union collective barganing agreements, and the government enacted right-to-work legislation I suspect that Unions would operate far closer to how they're supposed to.
