I suppose, at the end of the day, this is why things like morale rules don't apply to PC's. I believe that they should. I absolutely believe that PC's should be affected by Persuasion checks (or whatever the system uses) and the system should reward players who do so.
There's no way to enforce this. A PC can accept being persuaded and then change their mind back to their original position later on. A PC can accept that morale is broken and run away and then immediately restart their attack. If there's a reward or penalty, the players
will weigh the benefits of actually changing their PCs' minds or accepting the reward or penalty. Even in the most narrativist, most immersive game imaginable, they'll do this. And if the rules and penalties or actual gameplay are too boring or too unfun or get in the way of the players doing their thing, they'll simply switch games.