I don't know how Milton Friedman being extremely libertarian is a reason his criticism of unions should be taken more seriously. Libertarians (in the modern political sense, i.e. right libertarians) are generally anti-union because they want free enterprise and unions interfere with the freedom of business/capital owners.
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).
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).