Labor unions don't challenge the core structure of capitalism; even when they're working, they mostly serve to give a slightly bigger piece of the pie to workers. And in practice, they are hijacked by a certain bureaucratic caste that mostly optimizes for stability and self perpetuation. They become integrated with state sponsorship, which will never allow for substantive change. For instance, in the USA the general strike was a powerful tool in the arsenal of workers' power and drove substantial wage gains, but disrupted capital too much and as such is banned by the NLRB. Since unions' scope is limited to accounting, law, and mediation, they become mostly administrative organizations staffed increasingly by members of the professional services class. These people can never provide leadership that primarily serves the working class, as their economic interests diverge and they can't do anything that would threaten their social good standing.

Argument against:

An economy dominated by worker co-ops is just wishful thinking. We have no idea how to get from point A to B, and no idea if it would even work. The limited evidence we have for that kind of economic structure comes from post WW2 Yugoslavia and suggests it wouldn't ("they just didn't do it right!" invites the question of how we do do it right). Conventional unions do shift some of the capital pie to workers, and we shouldn't let a very hypothetical best be the enemy of the concrete good. And even if worker co-ops are the ideal, any path that gets us there requires more worker power, so stronger unions would be a good first step to getting us to that point.