The problem is that unions, in general, have expanded beyond what they were originally envisioned to do.
They’re not just a lever to make collective bargaining possible; they’re also tasked with investigating wrongdoing by or against their members, providing legal defense, lobbying the government, administering pension funds, and basically anything else that makes sense for the members to band together as a group to do.
And the problem is that most of that stuff is necessary. It’s not necessary that it be part of the union, but it’s necessary that the members have someone to do this stuff for them.
So, unless we can set up some impartial third-party agency(ies) to take care of these duties that are currently handled by the union, allowing them to return to their primary task of providing a collective front for bargaining, then we won’t be able to (and shouldn’t) strip the unions down to a point where they’re not overpowered.