Hey Thorsten!
You bring up an interesting point: if I understood correctly you say that we should start with the 4 freedoms and then show that not having them is bad.
So, this is a fine way to analyze a problem.
However, to answer you question more generally, the reason why anything we postulate must be proven somehow is that if we do not do that, we are then open to postulating whatever and then acting on it, waiting for people to disprove the idea. This is a form of an argument from ignorance, which asserts that a proposition is either true or false because of lack or absence of evidence or proof to the contrary.
So, we definitely cannot start by postulating the 4 freedoms, and consider them to be right until the point someone proves otherwise. This is just invalid reasoning.
I, for one, very seriously don't see why anyone should be entitled to, say, change a program as they wish by having direct access to source code. Why should that be a basic freedom everyone has to have? On what grounds? And remember - a freedom's other side is obligation. That means that the basic obligation should be to provide source code to anyone. Why?
The response that it might be convenient to some people is too generic. Convenient in what way? I now have access to GIMP's source code. I want it to do something different. How do I make that change in real life?
Whereas you suggest a scenario when there is no source code available. To which someone might respond - look, if there is no obligation to give away source code, that means people can build businesses with software development and I wouldn't need to change very much about the program because it will be of very high quality due to people being paid for it full time and the business catering to the needs of the customers.
The goal of discussing this is to outline why access to source code is critical. You don't start by positing this as a dogma and then telling other people to argue against it - and meanwhile we will be calling those who disagree immoral and tell them that they don't want to change the world for the good. This is not a discussion if ethics, this is ideological warfare at this point.
What I am trying to do is make it into a discussion and demonstrate that one can be a FLOSS supporter, a decent person who wants good in the world, and yet disagree with what Stallman says.