I would actually cite ASoIaF as a well-done example of the decoy protagonist (criteria for which I'll cover later) because everything Ned did and his subsequent death still had massive impact on the rest of the story. Literally, the rest of the story does not happen if Ned doesn't die at the end of the first book, so I would not say those 500+ pages were useless. Also, ASoIaF is less a story about individual people but an entire country and sweeping changes across its society. It just chooses to tell this story through the grounded perspectives of its many residents.
But to cover what I think would make the false/decoy protagonist work well? Make sure the protagonist(s) you're replacing them with is still a compelling character in their own right. Hell, even better, make them a more compelling character than the decoy [to clarify, the decoy still has to be compelling in the first place - don't make them boring or unlikeable because you're worried the audience will abandon the story after they're gone]. Of all the main characters, I actually thought Ned Stark was the most boring one (still liked him though) and I became far more invested in the remaining characters and how they responded to this highly influential figure's death. In addition to this, make sure the decoy's death actually matters to the story. Like with Ned Stark, don't just stop their contribution to the plot with their death, make sure that everything they did prior to this actually made a difference and justify why the story benefits from this plot development.
This is also how I felt with Kamina and Simon (even though I did understand Simon was intended to be the protagonist from the beginning, I at least assumed it would be shared with Kamina). Some people might disagree, but I found Simon to be a much more interesting character than Kamina, who seemed to just be the most generic type of "loud, hotheaded shonen hero" (which I don't consider detrimental as I'm pretty sure it was intentional) so I was pretty satisfied with Simon inheriting most of the spotlight after Kamina's death. Also, Kamina's presence and the influence he left on everyone is still felt throughout the rest of the series so his character didn't feel wasted. And TTGL has a good number of enjoyable characters who were able to keep the show interesting, so it's not like the show sufferred in terms of entertainment value because of Kamina's absence.