Following on from your emailer who asked why Styles turned on Cena and not Reigns before, why would they have anyone turn on Cena or Reigns?
Both guys are either "polarising" or straight up hated, so I don’t get how you achieve a believable heel turn by turning on them, ESPECIALLY with a popular guy like Styles who people will definitely want to cheer over the two in question?! (Another example here is keeping Rollins heel after a lengthy injury AND pitting him against Reigns?!)
Cena’s a great babyface, there’s just a certain segment of the audience (ie, about half of it) that doesn’t like him. That’s not Cena’s fault.
Reigns, however, is not a great babyface and there’s no significant segment of the audience that wants to cheer him, but Vince McMahon thinks he’s the biggest babyface on the roster, so that’s how they treat him. They think that if Ambrose wins the briefcase, for example, and turns on his buddy Reigns, then people will cheer Reigns and boo Ambrose. I keep hoping reality will set in soon, but I’m not holding my breath.