Actually, Charlie is pretty caustic and demanding and often motivates by needling. Brady was able to take the whithering criticism. Drew not so much. I can only imagine the relationship he would have with our underapprecaited #1WR these days, because Charlie would not be inclined to sit idly by while the OC got blamed for playcalling us into defeat while the $9M diva dropped, bobbled, tipped, alligator armed or otherwise failed to complete 80% of the throws to him. Either those throws would diminish dramatically causing all sorts of other problems for the QB and the HC and maybe even the owner, or Charlie would sabotage the SOB because he wasn't going down on his hard fought reputational sword to cover for him... Charlie is a hard nosed bottom line guy. He's also got a large ego and is fairly defensive and not inclined towards personal accountability when the finger points towards him. He never really forgave Tuna for humiliating him.
RAC is a different cat altogether. He knows the system inside out, but he's not an innovator. The innovation came via Bill and Ernie. He's the relationship guy, the hugger, the motivator, the passionate DC who cares about his men and extolls them to make a play. Veteran players do tend to love that guy, although that doesn't always translate into wins. Didn't as a HC in Cleveland because he couldn't kick the divas there in the ass, particularly on offense, and he couldn't even make tough personal and personnel choices (like not backing the GM who was and would continue to screw him over with ownership). Would RAC be an improvement here? Possibly, although on a team this young his methodology bight backfire over time. Players like Merriweather whose ego battles the discipline inherent in this scheme don't always respond to anything short of a threat. Sometimes they don't ultimately even respond to that...
At the end of the day though both Charlie and RAC are trying to do one thing, run Bill's system in KC... Whether they succeed or fail will likely depend on whether the players Pioli has assembled for them represent sufficient talent and can execute consistently and whether or not Haley (who has serious ego and temperement issues) impatiently gets in their way in the process - or Pioli allows him to.
Neither one was coming back here because they are each on some level still seeking validation beyond being Bill's coordinators.