1. Arthur Smith
Smith rose to being a hot head coaching candidate based on the work he did with quarterback Ryan Tannehill as the Tennessee Titans' offensive coordinator in 2019 and 2020. He was a candidate for the Lions' job, interviewing for the position as teams with openings all wanted to talk to him.
Smith of course ultimately got the Atlanta Falcons' job. After three straight 7-10 seasons, he was the first firing on "Black Monday" after the regular season ended this year. His misuse of highly-drafted offensive skill position players, taken in each of the last three drafts (tight end Kyle Pitts, wide receiver Drake London, running back Bijan Robinson) was his ultimate undoing. Not having a good enough quarterback didn't help, but Smith also had a hand in Desmond Ridder's failure to truly grow over the last two seasons.
The set of skill talent the Lions have built would have surely been similarly misused by Smith. And it got to a point where Jared Goff did not have good rapport with Campbell's initial offensive coordinator hire (Anthony Lynn), so what would have happened with Smith as the play caller? Smith may also not have wanted Goff, when the deal to send Matthew Stafford to the Rams was basically done and Stafford was allowed to choose where he went. Smith could have been a fly in that ointment.
If Smith had been hired as the Lions' head coach instead of Campbell, with his Falcons' tenure now as the evidence, he would have been more of a hinderance to success than a reason for it. But it clearly reached a point where Campbell was deemed the guy, so they avoided the mistake Smith would have been.