[hourago]

Many rights that everybody enjoys were won by unions and applied to everybody. You are generalising to all unions some hypothetical case.

Better job opportunities for a part of the population means that all employers need to improve conditions to compete for employees.

[seabird]

Have you ever worked in a union shop, or have family that did? Demarcation is not a hypothetical case, and it's not something that's so uncommon that you can just not worry about it, especially as a worker that needs to consider whether or not your opportunities are going to be hindered by it. Me, my father, my grandfathers, my friends, their fathers and grandfathers, and basically my community on the whole have watched it happen firsthand or have been directly affected by it. Union membership can be a big choice where you're heavily incentivized to join the union not just on the merits of them protecting you as a worker, but because they're going to work against you if you don't. Some unions have worse problems with it than others, and some don't have the problem at all, but it's not something that is so minor that you can just ignore it.