Nah, depends on the ability to coordinate. Communist governments can suppress wages even with enormous productivity gains, like post 1949 Shanghai. Capitalists mostly can’t though. Even with non state puppet unions completely illegal as in the PRC or Vietnam competition for workers leads to rises in wages. Maintaining a cartel without being able to enforce contracts to that effect is hard with fewer than 10 conspirators. Once you have more than 100 people bidding for workers’ labour it’s impossible.

Unions aren’t necessary for increasing wages. We can tell from countries that they’re illegal.

Unions are necessary, as we see when we look at the US. As productivity has increased workers are receiving a smaller and smaller piece of the resulting wealth. Not coincidentally labor is less organized now than it has been in almost a century. If you have power you get resources, simple as that. Individual workers usually have almost no power - they may get a little bit of power temporarily when their skill is heavily in demand but that isn't enough to counteract the inherent power imbalance capital possesses.

This is demonstrably untrue - looking at the tech industry for example. Salaries are enormous with no unions.

You shouldn’t state opinions as fact.

This greatly depends on which part of the 'tech' industry you're in. Yes, if you're at the top, salaries are enormous, but uh, vast majority of people in 'tech' are not receiving enormous salaries.

I think you've misinterpreted GP. Tech salaries started high, and that is mostly a scarcity problem. The market will correct that in due time without labor organizing because the profits go to the owners, not the workers. The owners are the ones who decide what happens with the profit (reinvestment, their employees' wages, their own wages, etc.).

Yes, with a powerful union you can restrict entry so there are fewer workers. The workers who are allowed in get a better deal. The people who are kept out by the union suffer and the consumers suffer through lower production and higher prices.

No need for that. The golden c64 generation will start with early retirements in a few years. Younger generations have less interest in IT (on average) and will yield fewer and fewer tech workers.

What I describe is the mechanism by which unions increase wages for their members. It would be good if you could explain what you were trying to say and its relevance.

Less workers means scarcity and implies better wages and conditions.

Yes. You get less workers by restricting entry. That reduces output but increases union workers bargaining power, making society generally poorer but the members of the union richer.

It's not necessary to restrict the number of workers - only to refuse to work if the owners allocate too little of the resulting profits back to the workers. This is in fact the whole point of a union.