I think you're getting it wrong:I don't claim, that we can't trust the distro makers, and I don't really believe that they replace some code to do harm to the users. But this possibility exists in theory, and that's a problem, since free software wants to give back the control over the computing to the users. For advocates of proprietary software, this possibility would be a strong counter argument.
I will read a bit through the documentation of Guix and find out, how it deals with the problem.
