At least historically, unions weren't just pure bureaucratic collective bargaining apparatuses. Many had strong mutual aid programs and emphasized skillsharing and so on. Also, there is a political element to them which acts as a counterbalance to other organized sectors of society, and even those who recognize the perversity of certain unions aren't necessarily going to make opposing them a high priority in lieu of their relative powerlessness.

Although it might be nice not to have lots of special interests fighting for privileges, as long as that is the case the order in which they are attempted to be dismantled is important.

I would consider the idea of unions to be more akin to a corporation than cartels. Some corporations are monopolistic or oligopolistic, and some aren't. Perhaps nowadays unions are worse than they used to be, but a lot of that actually has to do with state intervention in the way unions are allowed to be organized.

Just as state interventions such as corporate personhood and limited liability create perverse incentives within corporations, the Wagner and Taft-Hartley acts create perverse incentives among unions.

Though I'm really not interested in any of that anymore per se. In Henry George's book Protection or Free Trade he likens the idea of protectionism to one of a series of "robbers" who robs you every day on your way home from work, but says that as long as there is a final "robber who takes all that is left," there can be no benefit to eliminating the other robbers.

As Ricardo showed, with his Law of Rent, in an otherwise free market the Land Monopoly takes all that is left (save enough to keep you from starving). For this reason, right-libertarian reforms which ignore the Land Monopoly would take us to a Malthusian condition, though not for the reasons Malthus argued. (Ricardo roundly stomped him in debates)