Except the fact that at any given moment, the company only has $X to pay you from, and the union makes it so that they have some amount less: $X - n.

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."

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.