There is more than one union. You can also vote/organize to change your existing union (I can’t do that at my job).

Okay, so I get my one vote and now have to go spend time convincing others of something if I wish to change how I live my life. Even if there's less of a "safety guarantee", I'd prefer to take the risk of managing my own situation, something often thwarted by closed-shop unions.

Specifically union power tends to be rather exclusive - competition or fragmentation over a shared niche harms them far more than even individuals or businesses. The dynamic is thus "anti-competitive" by design.