> It is strictly harder to quit a union than to quit one's job. Because the latter not only involves quitting one's job, but also finding new employment in a different field.

(Did you mean "former"?)

I think you're being more than a little hyperbolic. Unions can't force employers to not hire you if you're not a member (in fact most employers probably would prefer non-union employees because the employees have less negotiating strength).

Not to mention that historically unions have been the catalyst for workers rights legislation (like the 5-day work week, and 9-5 hours, overtime pay, and so on). So even if you aren't a member there are obvious benefits to employees who aren't members of a union (in the long run).