Re: "he can pay you to develop it. Or pay you for setting it up on premises"
Some problems we can deal with:
"Can you see the failure of your assertion in the article case?"
Original assertion: "he can pay you to develop it. Or pay you for setting it up on premises"
"1) They don't need to pay you to develop anything, they develop it themselves and don't make it open, as the license doesn't require it"
So clearly, they won't pay you for that, because they paid someone else for that. Doesn't really change the math; you could have been paid for that if they chose you for the job.
"2) They can set it up themselves."
Once again, someone else is being paid for something you could have been paid for if they chose you.
The assertion said that you could attempt to sell further development or setup for money, not that people were guaranteed to provide you with work in that area. There are various services you could provide around an open source codebase, but there are several caveats about those. The primary one is that you would be providing a service that someone else could provide. For example, it would be completely possible for someone else to provide the kind of Linux support for which Red Hat is known. In that case, Red Hat loses. But Red Hat didn't lose, so it clearly works at least some of the time. Meanwhile, I run plenty of code that Red Hat wrote at some point, but I don't pay them for support (using Fedora/Cent OS/other distributions that contain some Red Hat projects, but not using REL). By making their code open source, they accept that some people will be like me, and they realize that this might actually be quite helpful to them later down the line.
"Anyway, the opportunity of making money by developing new features and installing disappear when everybody can obtain and install your software"
Not really. Plenty of people hire open source developers to put another feature in because the developer wasn't already planning to but they are most competent to continue developing on their own codebase. You're correct that there are many other options that don't result in the dev getting money, though. But the opportunity of making money by making people buy the software disappears when you make the software free, too, and we don't complain about that because the dev theoretically realized that when they made that choice.
"you can offer it at far lower prices when you don't have to pay for development also"
This was in the sentence with a discussion about developers, but I presume "you" now means the companies that sell stuff based around the software. And your point is? Lots of people don't pay for everything in their system. The raspberry pi probably would have cost more if they had to pay for development of their own OS to run on it. Instead, they ported Linux, requiring much less code. That resulted in more Linux users, more developers who can contribute code upstream, and a cheaper computer for us. This strikes me as a win-win situation, but your tone above sounds like you took this another way.