> teachers for example
Here's my take. The only way a teacher can get any legal protection is through the union.
In today's culture, who wants to be in a classroom without any legal protection? Any kid making a false statement about an assault because they're pissed about their grade, or a rich parent pissed because their kid doesn't get preferential treatment, etc., can ruin a teacher's life
There's a lot of teachers who aren't political. Union membership for them is not a political statement. It's the only thing offering a service they feel they need.
If the reason teachers are enrolled in and pay dues to a union are addressed, and service teachers feel they need are offered by other means which more economical or more effective, union membership would go down, IMO.
There are also a lot of asshole school administrators. A hard ass union rep is the only option for a teacher they try to target. Legal protection would help in that situation if it were provided by parties other than the teacher's union.