For some reason, this logic never seems to apply upper management salaries - irrespective of competence or achievement.

Here's the reality - the C-suite is already informally unionised. There's no explicit union structure, but there's a very cosy network of personal relationships, social signals, and ethical assumptions which relies on rhetoric like "We should pay talent what it's worth" to make sure that executive-level comp only shrinks in exceptional and rare cases.

Unionisation may not be the most perfect balance to that, but it's much better than any alternative which amounts to "Eh, let's just do nothing."

This is because upper management has an imbalance of power, agreed.

But it seems to me that unionisation ultimately gives management additional power - when the management can re-direct employees to the bureaucracy of the union structure - where the union has negotiated protection for their own upper management - rather than manage with them as individuals with individual circumstances.

But they don't already manage them as individuals as individual circumstances. They manage them as interchangeable pieces who have weakly singular leverage.