Milton Friedman is as libertarian as it gets, but he makes a good case about how unions only serve current employees, and have no incentive to see the business grow and serve & hire more people if it does not lead to higher wages for employees.
The alternative being lower wages for the same work? This argument doesn’t make sense to me, unless he implicitly agrees companies only grow by exploiting workers.
Unions can also use their leverage to mandate a better and safer working environment and reduce workloads, which reduces the short-term productivity whereas a business owner could otherwise weigh the benefits of these measures reducing churn (i.e. people being too exhausted/injured/maimed/burnt out to continue doing their job and having to be replaced) against the overall increase in productivity.
"Growth" means increasing profits. Profit means surplus from income that is not paid out to workers (wages, bonuses, etc) or spent on sustaining the business itself (operating costs, rentals, service contracts, etc). Payroll can easily be one of the biggest cost factors and productivity one of the biggest factors to income, so being able to squeeze both of these liberally makes it much easier to grow a company. This is completely tangential to the interests of the workers.
Unions in some other countries are less antagonistic )zero-sum)and more symbiotic (grow the pie).
But you're right to an extent: unions are a stop gap. The better option would be collectivization, turning the business into a cooperative so all workers have a stake in the success of the company and unlike shareholders have a strong interest in its sustainability, which may in some cases involve lowering wages (to weather a loss of income during a market downturn or to allow more room for investments, not to increase profits in quarterly reports).