He walked into that one, but the fact is, there are many sites that DO get most of their traffic from search engines, and they make a lot of money off of it.
(Spoken as a former SEO who could prove his clients' ROI was well in excess of his substantial consulting fees.)
So, if that were the general case then ALL seos should be able to prove to their clients that their ROI exceeds the consulting fees, in which case they can all work on a no-cure-no-pay basis.
The problem is proving that any change in sales is due to SEO activity and nothing else - "you say our 100% up turn is due to your optimisations but my wife reckons it's my new toupe, now those cable ads are drawing them in droves to the website ...".
If your sector fell by 10% but you only fell 5% did they do a good job?
(Spoken as a former SEO who could prove his clients' ROI was well in excess of his substantial consulting fees.)