Mack Brown won a national championship at Texas, not UNC. I doubt very much the administration at UNC cares about a title he won somewhere else. And despite the talented QBs you mentioned, how many ACC titles has he won since returning to UNC? The fact is, he's struggling. I'm not going to pretend I know what UNC is going to do. I have no idea how much they value football. But I do know UNC is not a great football school.
This is Brown's 6th season since returning to UNC. They've made bowl games every season thus far, but they've lost 4 straight in those bowl appearances. In 5 seasons he's finished with a winning conference record twice. They're currently 0-3 in the conference this season. So pretending like Brown is still a great coach is ludicrous. If UNC values football, it's not beyond reason that they could look to replace him after this season. It's also not beyond reason to think he may finally retire.