TL;DR: the key is to realise that you wouldn't be denying them a revenue stream. You would help them improve it. Look at it from a broader perspective.

If you say "You've projected so-and-so much revenue from this feature. Our projection is that it's implementation will cost this, future maintenance that, and it will introduce bugs which will lose you this much revenue and cost this much to fix. We propose you change the spec this and that way, and you will get out of most of the drawbacks while still retaining most of the benefit"

any sane business would not go "shut up and go home" but rather engage "okay, but what about sub-requirement xyz -- did you consider that in detail?" and from there on you can create something amazeballs together.