True enough, and as you say, Corporations run the same way. Where I think we differ is that I don't believe they were ever intended to be anything else. Labor unions were originally established by political idealogues in order to co-opt the grievances of the working class, however legitimate.

I think this is largely accurate, but even those unions who were originally intended to be something else would eventually become bastions of corruption and cryonism.

Any benefit they have provided the working class has been incidental

You have to benefit them sufficiently to want to be members of your organization. It's obviously not the primary aim, but this is quite important or the power-holders would find themselves with a declining organization. However, not incidental.

Public sector unions are the inevitable final form of the labour union: the co-option of working class political action by the pampered ideologues of the managerial class, to the point where the majority of the working class in the West are no longer unionized, but the bloated mass of social-climbing hangers on, clinging to the teat of the globalist elite, are the chief beneficiaries of labour unions. Which is further proof that they have only ever existed by the leave of corrupt elites.

I don't think it's true to say that in the beginning, they had the support of corrupt elites, but now since they benefit other corrupt members of the corrupt elites, they certainly do.