By all measurable accounts, Cahoon's first stint was successful. See this article from Cahoon's first stint here:
https://www.ksl.com/?nid=498&sid=23142343
The only reason he was let go the first time was because Anae wanted to get rid of Doman's staff. And at the time Anae came back, the prevailing opinion on CB was that Cahoon was the one guy people were hoping Anae would retain.
But then Guy Holliday tweeted in all caps and stuff, so he was clearly a much better coach.
Obviously the receivers haven't been great this year, but I'm not convinced that BYU has much talent at the WR position right now. Not sure you can put it all on coaching.