If we're approaching this strictly from a reward balance perspective, then I'd agree.

But by doing so all it accomplishes is sacrificing a satisfying story by intentionally sabotaging its outcomes for the sake of imaginary reward balance, which doesn't really even solve anything as all it does is create a solution for a problem that doesn't exist. Especially considering the amount of extreme obstacles required to be beaten to even achieve this piece of dialogue, let alone fully succeed it.

Keep in mind that the player has to go through all of these extreme obstacles in order to even get a chance to experience the interaction all the way through:
Quote
  • Getting Auntie Ethel below 15hp and keeping her there until she gets her turn is incredibly hard as any class can easily overshot 15 dmg per hit.
  • The risk of failing to obtain both Mayrina and the Power are quite high.
  • You do not get to choose who does the talking as the dialogue is proximity based mid-combat.
  • The Deception check is 20, the highest possible value and is supposed to be a false promise by all means of the word.
  • You cannot apply any buffs for speech (such as Guidance or Shadowheart's Enhance Ability to give you an Advantage) because you are still in combat during this scene.
  • You only have the Charisma bonus, Inspiration points and most of all, luck to succeed this roll.
  • And if you want to save the 4 masked people, you also have to survive and ignore them the entire fight while they try to kill the party

You are more likely to;
  • Kill the Hag without ever seeing this dialogue as 15HP is easily overshot by any class in the game. Barbarians/Fighters/Rangers and Rogues especially do insane numbers with just one hit.
  • Fail the required speech check than succeed it as it happens during combat
  • Get Mayrina killed by the Mask Of Regret's AOE if you are also trying to save the 4 masked people
  • Get your party killed

So why should after all of these extreme obstacles the player be punished with reward balance by sabotaging the deception outcome with a non-satisfying ending where the Hag survives? If anything that will just make me ignore all of these requirements and just kill the Hag.

It is very important to understand that the whole reason why I even came to submit this feedback is exactly because this imaginary reward balance directly affected my story choice and broke my character immersion by not offering me a choice to finish the Hag after all the exhausting work I went through to get to that point. And most importantly because it does not respect the meaning of deception.

So whether we should or shouldn't be allowed to obtain everything because of balance or how other players might play the game is irrelevant because the rewards are just a bonus, not the primary purpose of encounters. The primary purpose of encounters are the story and outcomes. Story should never suffer for the sake of balance nor adjust itself to it, but the other way around.

And I have to also point out that practically the same situation is already present in the game, but this one actually respects character immersion:

Quote
A completely similar situation is with Fezzerk in the Blighted Village after telling him he might live if he talks. After we've beaten him, extracted all the information and forced him to hand over his valuables, we get to end his miserable existence.

If we apply the logic of reward balance into Fezzerk's example, then we shouldn't be able to kill him after getting all the information from him and his loot. He should just somehow run away. Which just isn't good storytelling then and why I think balance has no place when it comes to story outcomes.