Bandersnatch is working in the archetype of the addictive meme, similar to “The Parrot” (feel free to google the image, it is safe, hard Sapir-whorf doesn’t exist). These stories are stories where a compelling text or meta-text compels the focus character and threatens to compel the reader. They often involve recursive infection and infection “unto the seventh generation.”
As observed Black mirror don’t do happy endings. Well, they don’t do happy endings inside the text, usually. But Bandersnatch cues us early on about the infective meme. You can read this as early as the “do you understand how choose your own adventures work” screen where you are compelled to respond in a preprogrammed manner. The text offers us a solid clue (cue) here about how to read a happy ending: cease permutation and reject the text. A similar situation occurs when you pour coffee on the computer before anyone has started dying. This is another clue to reject the text.
Consider, as a relevant equivalent text, how Undertale deals with the reader recursively reading the text. Even if a spoiler switch isn’t set to 1 and the game is completed, the ending will indicate that the player will probably attempt to reread the text with the spoiler switch set to 1. Each ending is necessarily as horrific as the worst end because the reader will reread the text for the plurality of endings (or at least be made aware of this in Bandersnatch through the rerouting ending system).
Thus, the only winning move is not to play. The only happy ending is to stop the media file early, hard, and never return to it. In this case Black mirror have made a happy ending possible. It is Black mirrors’ viewers who make the unhappy endings inevitable.