You might think that getting a CFP appearance would help us get #4 anyways, or (backing up a little more) getting #2 would help us have #1, #3 and #4 but in my thinking, getting into the Big 12 has been such a moving target for BYU that we might have a great season with a CFP appearance and still not get in.
So, locking in BYU as a Big 12 member would help improve BYU's recruiting, maybe not to a #1 class but if we were consistently getting top 30-40 classes over many years it would impact our fortunes more in the long run than a single #1 class would. This would also improve the likelihood that BYU does tear off a winning streak vs Utah (if we've been that close in recent years with Utah despite their having a recruiting advantage due to their membership in the Pac12, improving our recruiting year after year should nudge us over the top.
Also, consistently recruiting in the top 30-40 with maybe an occasional class in the top 25 moves us into the conversation of a CFP spot.