You really think any of those people feel like they don't have the opportunity to stand out? Or that their financial upside is too limited?

Don't generalize about what a union can be based on what unions for one particular type of workers are. When a group of workers' labor is easily replaceable, of course their union is going to focus on things like uniform pay -- that way the workers won't get trapped into a downward spiral by demands that they undercut each others' wages or lose their jobs. But unions for workers who aren't easily replaceable will have different priorities.

I worked in an industry with notoriously low pay, and I felt that the union removed my ability to negotiate my pay.

Meanwhile, my workplace was paying for (and I was stuck dealing with) union-protected incompetence.

My salary is actually higher that the minimum (established by collective agreement) for my job. The union just negotiates the lower bound.

If your company values you more than that, they can always pay you more. Or promote you. Nothing stops them to do so.

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.