> Unions are intended for industries where a worker doesn't have a marketplace of options as there is a single or just a few employers (e.g. Hollywood, teachers, coal mining towns, hospital staff, etc.)

Unions serve a purpose of being able to collectively bargain, regardles off how many "options" there are.

Employers and corporations always have bargaining power and are basically collective establishments themselves. Individuals rarely have any negotiating power for better conditions, wages, treatment, etc...