I can't really imagine a client describing "I want to be able to replace any part of the system I would like, and it should be easy to replace" as an exceptiona

I can't really imagine a client describing "I want to be able to replace any part of the system I would like, and it should be easy to replace" as an exceptionally simple requirement and being taken seriously. In fact, I think only some open-source operating systems even come close to that goal.