I would guess you've never browsed through clientsfromhell.net.
Being honest, polite, and knowledgeable doesn't change the fact that the person who hires you may be ignorant, too busy, dead set on cheating you, or needs to assert his/her sense of control by insisting on changing something you considered finished.
that may be so, but tricking them into making the right decision is not a good long term decision. If they can't respond to reason perhaps changing teams and companies is a better approach.
"Quit your job" is not a reasonable solution to intractable problems. The duck isn't either, but the change jobs mentality is even more damaging than a duck.
Being honest, polite, and knowledgeable doesn't change the fact that the person who hires you may be ignorant, too busy, dead set on cheating you, or needs to assert his/her sense of control by insisting on changing something you considered finished.