Most agree that it will help recruiting and most BYU fans thought not being a P5 hurt BYU's rankings before.
So if you take a coach who got certain results without being a B12 member, wouldn't you expect at least slightly better results as a B12 member? And "slightly better" seems to fit your definition of success.
What compelling reason do you have to suspect otherwise?
For the record, if Holmoe can find someone better, I'm all for it. I don't know enough to do his job.