The thing I try and remember is that Cooper wasn't always an on-field assistant coach under Rhule. At Temple he had a Director of External Operations role, and after a year at Miami, he came back with a Director of Player Development. It wasn't until Baylor that he got the chance to have an on-field role. Before that time, Rhule had coaches like Fran Brown working the secondary along with Phil Snow coaching safeties. If Cooper was that big of an asset even back then, it's possible to still replace some of his capabilities in a separate position.
At the time of his resignation, Cooper had definitely evolved into a multifaceted component in Rhule's operation, leaving him without A. a defensive backs coach along with B. a film-burning recruiting wunderkind.
I think it's fine that Butler's hire is a damn perfect solve for just A. It could very well be an upgrade there. And it also potentially solves for an eventual C. when Tony White leaves.
As for B., this hire didn't need to solve for that. Rhule could make additional hires to attempt to replace Cooper's recruiting eye. Maybe the team is at a point where they no longer need to look for diamonds in the rough. Or maybe Garret McGuire sees this as an opportunity to take on a bit more responsibility and start burning through more Texas high school film. That's an assistant that will likely be seeing additional responsibilites through his career. Maybe Rhule hires Bill Busch as an assistant special teams coordinator or assistant defensive backs coach, or just general "defensive assistant" title and gives him Foley's recruiting hallpass.
And/or perhaps Rhule hires a couple more high school head coaches and works them into his on-field assistants with the intention that yes, these guys have headsets during games and practices, and will likely become full time assistants under Rhule in due time, but they're primary directive for now is to become experts on every god damn player in the state they came from.
Cooper's departure can be a loss, but there's multiple ways to make up for his departure. Butler looks to fix a big portion of that and fix an eventual future problem. Meanwhile, the 10-assistant ceiling being eliminated gives Rhule 1,000s of ways to solve for B.