unions don't measure productivity. They group people based on credentials + experience, and treat everyone as interchangeable in those buckets.

That's one possible union architecture, sure, but you're missing the forest for the trees. A union offers job security to reduce the risk of "managing up."

> one possible union architecture

It's the one used by all the major ones: airlines, auto manufacturers, and teachers. What institution do you know of that has a greater emphasis on measuring performance?

A union's priorities reflect those of its constituent members.

You're thinking of management.

I'm sure most unions would love performance based pay where union reps decide what constitutes good performance. For some mysterious reason management is as keen on unions exercising their judgement as unions are on management deciding.

Where unions agree policies that treat members as interchangeable (e.g. age based pay) it is usually as a result of a compromise brooked with management who would love to have the latitude to give pay raises to scabs, kiss asses and spies.

That's not generally how it works in professional sports with player unions. Nor does it work that way in the film and television industries where there are unions representing writers, directors, actors, etc...

The shape of the union is whatever the membership wants it to have.

Both of those examples have weaker unions that play less of a role. Also the poster above suggested that the union as opposed to the management could measure performance. That doesn't happen in sports or TV.