Now how do you convince graphic designers and marketing people that this matters more than "branding" or "impact" or looking "cleaner" or any of the other vague

Now how do you convince graphic designers and marketing people that this matters more than "branding" or "impact" or looking "cleaner" or any of the other vague feelings-based reasons they give to deliberately make the user experience bad? I'm constantly dealing with people who insist on making the user interface bad because they think it is part of their branding and that matters more. And no amount of explaining "you are making less money by having fewer people able to use your app" seems to convince them.