I'm sure Souths could improve with better coaching, but their roster is on the slide and that's not readily fixable. Cook is 32 and turns 33 this season, Walker is 34, Tom Burgess is about to turn 32 and is gone after this season - that's half the spine and their only impact prop. They don't have a halfback, they're paying a million dollars a year to Latrell Mitchell who while talented is nudging towards liability status. The prop rotation is thin, there isn't a decent winger at the club and the depth is so nonexistent that Michael Chee Kam is getting named every week. Could they get their shit together sufficiently that they make finals this year? Sure. But "top four team"? You're dreaming. Their roster as currently constructed is no longer a contender.
If an up and coming coach goes to Souths now they're taking a gamble that (a) the roster can be turned round fairly quickly, and (b) they'll be given enough time to oversee it and come out the other side rather than being scapegoated in the meantime.
(A) is possible: if nothing else, Souths have an almost Roosters-level ability to get players to walk away from the out years of terrible contracts. If Mitchell can't get his head right watch out for him to take the Greg Inglis approach and "retire" rather than pick up the $1m a year he's owed by Souths until 2027. On the other hand, if the club is looking at Wayne Bennett it means they still think they can win now, which would make me very worried indeed about (b).