You are right. But it saddens me the lengths you have to go through to explain this (although I do think your explanation lacks clarity).

I see this issue is very simply taken care of by simply conveying the notion that it’s part of the job of the designer to make these decisions. Just ask your client: Would you advise your surgeon on what type of scalpel to use? Are you sure you are in a better position to make a more informed decision on that subject, while your surgeon is the specialist and while you’re bleeding all over the floor?