HHH isn't the only person who knows anything about wrestling. Everyone is replaceable. People thought the same about Vince for a long time and now that he's gone they're still thriving. I know how much people love HHH around here but he is replaceable. Just like Vince was. The brand is to big and most people aside from wrestling nerds like us won't be able to tell the difference of who is in charge.
If or when Hunter is out, I would bet TKO will lean on Rocky to determine that. Remember he has Brian Gerwitz on his squad plus a whole team of hollywood writers. That is the most likely scenario IMO.