That's entirely true. But you're likely to sell very few copies, because all another company has to do is buy it for the price you ask, and redistribute en mass

That's entirely true. But you're likely to sell very few copies, because all another company has to do is buy it for the price you ask, and redistribute en mass. All they have to do is make up the price they paid, you have to worry about the time you put into it. So what ends up happening is reduced quality or higher prices. (Basically, everything being a bespoke solution, because you aren't going to be selling too many.) The GPL is a poor model for something like a wordpress theme.