It's not the same argument, and here is why:
Whether or not it is real to you or me is irrelevant to whether or not it is "real" to a character within the setting.
In the setting of Galaxy Quest, Thermians believe in things which are objectively (in-setting) known to be false.
In the setting of Forgotten Realms, the Wall is connected to concepts which are objectively (in-setting) known to be true. It's just as real as dragons, elves, and magic.
I would posit that, if I approach the game from the stance of everything being fake, the PCs (and the players controlling them) are always the villains. I posit this because -if the assumption is that everything is fake- it would mean that there are no such thing as monsters, and the players are engaging in a mental exercise of murdering other beings for the selfish reasons of personal gain and gratification.
I notice that (I think) you edited your comment to add that the Wall may push some players away from the game. I'll note that I addressed that in an earlier comment.
I can see how the concept may rub people the wrong way. Alternatively, I can also see it as a built-in point of contention and conflict to help drive the narrative and motivate characters in a roleplaying game. There are a lot of ways the concept could be creatively explored through narratives.
Personally, I don't have an emotional attachment to Forgotten Realms, so I don't care if the Wall exists.
I think you miss the point trying to be made.
In a lot of ways, Fiction must be more real than reality, because Fiction is intentional.
We actually see this a lot in media that we end up disliking. Why is Hans a bad villain in Frozen? Is it because it is completely implausible that a man can emotionally manipulate a woman for his own self-interest? No. But, we as the audience don't see his betrayal coming, nothing really strongly hints at it, it just... happens.
Imagine for a moment that you are reading a story about political intrigue. A new, young King of a deeply troubled state. You follow his idealistic journey through politics, his trials as he finds nobles he can trust and begins to build a better nation like he promised. And then in the last chapter he falls of his horse, breaks his neck and dies, undoing all his work and leaving none of the plot proceeding it to be relevant.
It feels dissatisfying, it feels wrong. Why did we follow this story if it was all pointless? Is it "realistic"? Sure, but Fiction can't be random, it is written intentionally.
And so, it doesn't matter whether or not the characters see the Wall as Real, because we know it was intentionally created by an author outside the setting. And we know that that author could have chosen any set-up, they could have altered every aspect of the world to fit a more coherent vision.
It is, in the end, a choice. Unlike reality.
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From what I've read over the past day or so, the god of death was pressured into building the wall by other gods. Some of the other gods noticed that mortals had started to ignore them and/or* to only worship the god of death. Essentially, the gods of Forgotten Realms were (as I stated in an earlier post) kinda d-bags.
*The few things I've read aren't consistent on the exact story.
I'm not arguing that the concept is written well -or in a logical manner. However, that can be either a bug or a feature, depending upon point of view. In my head, I have ideas about how I could use the general concept, around which to build plot and adventure. But, as stated several times, I have no emotional connection to whether or not the Wall exists.
I understand you don't have an emotional connection, but I would like to correct your misunderstanding.
The events you describe about the Gods pressuring the god of Death? That really had nothing to do with the Wall.
You are talking about Kelemvor, who had offered a paradise to those souls he judged worthy, whether or not they had done things the way their individual gods had wanted. This supposedly caused problems. As part of that intiative, he stopped using the wall.
Then, after a stupid "revelation" you ca read a few posts back, he reverses his decision, coming to the conclusion that if he rewards good people in death, then everyone will suicide and die if they are good, but remain if they are cowardly and evil, and thus the world will fail. As part of him going back to the old ways, he resumes using the Wall. Specifically sending a High Priest into the Wall, because another God drove that priest mad, and that priest then denounced the existence of the gods before his death.
But the actual creation of the wall occured easily 1300 years before those events, when the God Myrkul... just created the wall. For no reason except to be cruel. And, none of the gods pressured him into not doing that, while they did pressure Kelemvor.
And this is our fundamental problem, the wall seems to have been arbitrarily created as a torture device by an old god. But now it is being billed as a fundamental aspect of reality which should not be challenged.