Most of the unions I've seen aren't voluntary. When I was in grad school, I was forced to pay dues into the union. I don't like that. I think there should be as many unions as employees like. I don't understand how the union can be understood to be negotiating on my behalf if I can't choose not to have them negotiate on my behalf.
Once the union has negotiated a budget for itself, it has an independent mandatee to sustain itself, which is sort of detached from any selective pressures. If I'm a bad employee, I get fired. If the company is bad, it goes bankrupt. If a union behaves badly, the consequences are indirect. A bad union only gets its comeuppance if the company goes under.
Because the selective pressure on it is indirect, evolution can't happen within regimes, it can only happen when union workers transfer (or don't) organizational knowledge into a new host company. Basically, like a virus. It's just weird to have an entity that doesn't face any consequences.
To the extent that employees have collective bargaining power, they can just use it. Get a group together, write a letter to Elon, and threaten to quit. See what he says. If you have any leverage, you can use it. If you don't have any leverage why would the union fare any better?