One of the problems we seem to have is that there is still belief in the "trickle down" effect; the idea that if we give rich people shedloads of money, it'll eventually end up in the hands of much poorer people.
Part of the problem with this is the perennial problem that rich people, compared with ordinary people, don't spend their money. Presumably that's part of how they became rich; by holding onto money. From personal experience, I've had months in which my own pile of money makes more money than my full-time job. I expect if you're rich enough that this can happen every month, the temptation to not spend it becomes immense.
One of the benefits of unions is, as you suggest, getting a decent wage. This wage comes out of the company, and one way to do it is reducing the ridiculous CEO salary and other such big salaries at the top.
So now we have a lot of ordinary people with money. What do ordinary people do with money? They spend it. This is exactly what we want to happen. They go out and they create demand for consumer goods and services. In effect, they create jobs. A thousand ordinary people spending all their money creates a lot more demand than one rich person dumping it into an offshore tax haven.
Sometimes you hear the rich describing themselves as job creators; they're wealth sinks. Money comes to them, and stops (or at least radically slows down). What creates jobs is demand for goods and services, and the ability to pay for it; unions ensuring decent wages directly create this effect.
As cruel as it sounds, are workers not a commodity?
This, to me, is something of a moral issue; I accept that you can take a strict economic view of people just as one part of the economic machine, and the point of all creation is to create and run an economic machine. I take a different view; that this idea is repugnant and grotesque.