[geofft]

> I don't know how effective they are in practice. If retail workers tried to unionize what is to prevent the millions of people who are equally qualified from replacing the entire union?

See the strike of grocery workers at Stop and Shop earlier this year: https://en.wikipedia.org/wiki/2019_Stop_%26_Shop_strike

They are effective in practice.

(Some possible first-principles reasons why this might be true: training replacement workers is hard and the labor market is not quite that liquid; new workers will want union representation too; new workers will want good wages too; the fact that the Teamsters respected the strike and union truckers refused to deliver across picket lines meant they'd have to find replacement non-union truck drivers too.)