It is worse than them not paying you. "You" (the company) are paying them. That means you want to minimize the amount of time they spend on the software without getting further returns of some sort.
The point I was making was that if your product is good, even if the source code is terrible, you can still keep selling it as-is and keep making money. You won't be able to improve it easily, but at least the current state is producing value. By contrast, if your product is bad, then you're going to miss out on revenue until you fix that, no matter how awesome your codebase is.