I think it was that Bubba knew from old time football boosters that perhaps the only 'big name' football coach that UNC could hire who would not be undermined almost immediately was Mack Brown. Mack was willing and able to charm the UNC old guard, including much of the part of it that was hostile to 'big time' football. And they all loved Sally.
Even Art Chansky, who at times comes close to confessing his abiding distaste for football, genuinely likes Mack and loves Sally.
That is the realistic reason to be hesitant to tell Mack to retiree, even at 75. The UNC people who truly wish UNC football to be lesser than basketball will demand a search that finds another Bunting or Hickey or Withers. But an aging Mack who is fiercely loyal to mediocre assistants also will end up harming the program.
So what is Bubba to do? He has to walk a fine line encouraging, and then incentivizing, Mack to clean house to try to field the kind of staff that can win the ACC and make post-season noise.