“No 4, No HD” theory:
Happiest Day did not happen because we did not solve FNaF 4
# # #
This is a copy-paste from a Reddit post
Yes, you read that right. We failed to give the MCI kids their happiest day. We doomed them, all of them. They were forgotten; their memory locked deep underground, restless. The consequence?
Scott's Steam Post and FNaF World
In the Halloween Content Update post on Steam, Scott talks about the box and how the community couldn't figure out FNaF 4's story. He said we couldn't find the pieces and that they remain elusive. He said the box contained the pieces put together. What are these pieces? We learn that in FNaF World.
In the secret ending of FNaF World, Yellow Eyes guides the player to put the pieces in place for BV to find. What are these pieces? They're the hints seen in Follow Me for accessing FNaF 3's secret minigames. Let me clarify, the pieces are not the secret minigames; the pieces are the hints themselves.
Scott said that the pieces can be found in FNaF 4. Years later, the community linked the random easter eggs seen in the FNaF 4 minigames to FNaF 3's secret minigames. But that's the issue: the random easter eggs are supposed to represent the hints!
From the Past to the Future
The method of obtaining the hints during FNaF 3 is very meta and adhoc. FNaF 4 fleshes out this method by introducing the tragic life of the Bite Victim. In order to access FNaF 3's secret minigames, one has to look back at the Bite Victim's final week leading up to his party. One could say... look back through his memories.
Who is looking back through the Bite Victim's memories? The older brother, Michael Afton, the person who is going through the FNaF 4 gameplay: a mixture of Michael's own memories working at Freddy Fazbear's Pizza and the memories of Bite Victim's nightmares of the animatronics. This is what the box itself is all about: confronting past trauma in order to move on. Michael is confronting his trauma of killing his younger brother in search for the pieces that'll allow the MCI spirits to finally have their happiest day and be freed. Facing such childhood trauma is a difficult task at hand, which is why some things are best left forgotten, for now.
In the end, Happiest Day is about celebrating the birthday that never happened. It's about turning the tragic memory that is the Big Bite, into a happy memory. This is exactly what Jake does to free Eleanor's victims in Fazbears Fright, and it's what Oswald does with the Ball Pit's memory of the MCI in Return to the Pit.
The Community's Failure
But we never found those pieces, did we? We never properly found out how to access FNaF 3's secret minigames. We never gave cake to the MCI spirits. We never celebrated the Happiest Day. We never achieved the Good Ending of FNaF 3...
Scott found the community's inability to find the pieces to be a fitting conclusion. He made the final decision in that same post: "some things are best left forgotten, forever." We got the Bad Ending of FNaF 3, and this is the change Scott would make between FNaF 4 and Sister Location. This is why Animatronica is getting corrupted by Scott; because the story of FNaF is now changing. In Update 2, Scott made something terrible; Circus Baby, the representation of the story's new direction. Our failure lead to the creation of Ennard, and in turn Molten Freddy; a physical representation of the Bad Ending of FNaF 3. The only way to free the children now is for them to burn...
Why can the box never be reintroduced back into the story? Because the Good Ending of FNaF 3 is no longer canon. The pieces were never found and so it's impossible for that ending to happen now. It's already too late, because Molten Freddy exists.
Now, what happened to the Bite Victim? I think I'll save that for another time...