Do you have any data to back up your claims? Because from what I understand, unions were the backbone of the recovering US labor market following the Great Depression, which punctuated a period of extreme speculation and inequality (the Gilded Age) similar to what we're seeing today.
The data I've seen largely show that union membership is highly correlated with better pay, benefits, and working conditions. It seems clear to me that individuals are at an extreme disadvantage in wage/salary negotiations due to the enormous information asymmetry present when the counterparty is a corporation, and collective bargaining seems the most straightforward way to reduce that asymmetry.
As far as I'm aware, the reduced effectiveness of unions in the US is a direct consequence of union-busting legislation at both the state and federal level (e.g. so-called "right to work" laws, Reagan's actions against unionized/striking flight controllers). If that's the basis for your conclusion, I would agree in the sense that we need a strong political push to undo anti-labor legislation to ensure unions can be as effective as possible.
I'm not advocating for not having unions, but the idea some people have and corporations push is based on a hypothesis that companies will try their best to minimize costs.
Unions do have these real benefits for the workers who are a part of them, and these benefits come at the cost of the corporations who have to pay more and implement better practices.
This increases the cost of human labor, so when doing a purely greedy analysis of costs vs revenue, the costs go up for processes which requires human labor, which drives down the profit, disincentivizing investment or even the continuation of processes that require unionized labor. Overall, there winds up being less demand for human labor and more demand for automation.
I'm not here to speak on the truth of this, I think of it from this perspective: Information asymmetry (not understanding the opposing argument) benefits the party who has the information, so understanding their argument is necessary if you want to counter it.