Shanny is known to be extremely meticulous and demanding with his coaches. Guy like Rex has been eating donuts and talking smack for how long now in the media, would he come in and log long hours and produce at the required level. I doubt it. Vrabel meanwhile got fired in part because he was supposedly at odds with the GM. That sounds like his ego might need to cool a bit before he'd be willing to get in line as a coordinator and submit to all that.
Compare either of those guys to a hungry positional coach and I'd say it's more likely the latter would give Shanny what he wants. Also and btw I have been saying all season that SF's defense doesn't look as dominant. They put up great regular season stats but got barbequed by the Lions and couldn't get the stops when they needed them in the big dance. Detroit should have knocked them out tbh. So I think the Wilks firing is appropriate, and that's a shame because I'm all for them having a less than optimal DC.