The thing you're ignoring though is that main way you reduce the power of labor is by increasing its supply.
For instance one of the key factors in society escaping feudalism and moving onto market based economies was the Black Death. It absolutely decimated society and the labor pool. This gave labor the power to demand more compensation than a share of what they produced. But in times before if they tried that then nobility could simply have said no, as there were plenty of peasants willing to work for little more than food. But when the labor supply was suddenly cut in half? Now they had all the power in the world.
Labor unions can't really combat market forces. I don't even think ethical or moral arguments work either. If somebody, in the country legally, is willing to do your job for less money, and is capable of doing so, then by what right do you have to insist that you should be the one doing your job and getting paid more? It doesn't really make much sense. If you want to increase the power of labor then, by far, the easiest way to reduce so is to reduce the supply of labor. And vice versa for weakening it.