I find this difficult to believe, given that I know people who've worked in admissions at a surprisingly large number of colleges-- private and public, universities and liberal arts colleges, elite and average-- and they've all indicated the importance of yield. It's not something that will get a mediocre applicant in, but it's easy for a good one to get rejected on low predicted yield.
This is one of the problems I have with college admissions consultants. They charge kids an arm and leg to help them get into elite schools, but few if any of them actually ever admitted kids to elite schools. All they can do is look at the black box, see what goes in, what goes out, and then make educated guesses about why. That's not good enough for the prices they charge.
I would argue with you on this one. The outcome can be altered, but most people don't have the means or knowledge necessary to do so. For example, consider a student with a 3.9+ from a reasonably good public school, and a 2200 on the SATs, and otherwise middle-of-the-road "soft factors". He's not getting in, obviously. If he improves his SAT scores to 2300-2400, or spends three times as much time on his admissions essay, it won't matter in the least. He's still not getting in.
On the other hand, if he had attended a prep high school (it's ridiculously easy to get into elite colleges from a brand-name prep) he would have a guidance counselor with enough social connections to state his case personally in the admissions office, and he'd almost certainly get in. Or, if he had a great internship-- especially of the kind that can only be bought, in high school, using elite connections-- he could get in.
The service provided by "admissions counselors" like Kat Cohen is the sale of their social connections: an internship at a NYC art gallery in your junior year, a recommendation from a Senator, etc. Everything else they provide is pretty useless. The reason this business model begins to fail at some point is that connection-pimping doesn't scale. Kat Cohen can do this for 3 students per year, but if she's pounding her network for 100 students, her "connections" are going to become annoyed at being sold to rich high schoolers, and fade away from her and her efforts.