In a world where antitrust law punishes companies for being competitive, I suspect the DoJ would instantly be down the throats of any companies actually colluding

.
Someone before argued something to the extent of "since corporations are a representative of capital, why can't workers have similar representation?" That misses the forest for the trees. Even in specific fields, there are often many competing representatives of capital that limit any supposed monopsony power. Moreover, as long as my knowledge is not so specialized that I can literally do nothing other than what I am doing presently, there will be competing representatives of capital in other fields. Still, actual competition isn't a prerequisite for competitive behavior (see: Standard Oil's pricing strategy in its heyday), so the main limit to the exercise of monopsony power is the potential for competitors to sprout up and take advantage of exploitative conditions by paying the workers a little extra.
Another natural check to exploitation by the employer is that the worker can just simply leave if he feels that the conditions aren't commensurate with his input. Because of this, the employer can only preserve an exploitative arrangement if I am able to be compelled into remaining where I am. This could really occur only through providing me with extremely specialized knowledge and a restrictive work contract, but both of these things would immediately be apparent by the point of hire.
Ultimately, though, you don't need to buy exploiting labor largely being a myth to believe that powerful unions are a destructive force. The socialist, redistributive fantasy from capitalist to laborer is the more important myth, as it's only a transfer to
some labor. To make above-market wages, the unions must necessarily 'close the doors behind them,' excluding other labor and including themselves as long as possible; in the long run, this depresses wages and productivity in the nonunion sectors and often productivity in the unionized sector (through restriction of competing capital, e.g. labor-saving machines). Recognizing that the purpose of production is not employment, the considerably more damaging loss comes as a rent extracted from the consumers.