The prison motif that inspires the early portions of the film plays a key (pun intended) role in the final sequence of the movie, as Imogen Rainier (Caitlin Gerard) ventures into the recesses of the franchise’s trademark location, The Further, to rescue Elise (Lin Shaye).
The climactic showdown with the KeyFace Demon in Insidious: The Last Key is already incredibly unnerving, but this would’ve elevated it to an entirely new level.
In the theatrical cut of the film, they simply walk out of the otherworldly realm once they (or rather, the spirit of Elise’s mother) have vanquished the monster, but in the original cut of the movie, the moment caused a series of doors to open so the trapped souls of the spiritual prison could rush out.