Well, we don't know what the other characters do. Film crew presumably leaves too, but the equipment can stay and keep rolling for some time. I'm thinking either we just hear the monster off-screen running around a house opening and closing doors screaming "where are you? I wanted to eat your liver! I wanted to make haggis from your stomach and then sew it back into you so you would be technically eating yourself!" or something, if we want to keep it interesting. Then maybe the monster goes and sits down in front of the camera and cries about how they just wanted to have a breakout horror role and draw some tourists to the haunted house to eat them, maybe get cast in 5-9 ever-worsening sequels. Eventually the monster falls asleep in a puddle of their own tears, or crawls offscreen and then the last half hour of the movie is nothing but ambient noise and a shot of the same scene. A mouse may scurry by. Perhaps a beetle. Maybe some ants but they'll be out of focus. The true horror lies in being alone with our thoughts, bored, deprived of stimulation but watching intently just in case something happens anyway.

Movie critics will be polarized. The real artsy freaks will call it avant-garde, genius. Some small SlopTuber will go viral hyperbolically criticizing this film as The Worst Movie Ever Made (a ludicrous claim in a world where Traxx (1988) existed!), and parasitic copycats will try to ride their coattails. A few shrewd investigators will recognize that there is no deeper meaning or message to the movie, that it was not experimental art, but that the director probably just decided to fleece investors by making it as barebones as possible so as to abscond with half the budget after paying the actors, set designers, costume makers, camera crew, etc. for their work. Not the writers though, because it was written by the director. By the time the warrant for the director's arrest for embezzlement and fraud has been printed, she'll be long gone, hidden away in her underwater lair and using the profits to establish schools for the denizens of the sea (especially cephalopods) to teach them English, basic computer literacy skills so that they can type, and theory so they can go forth on the Internet and spread a holy message, a particular synthesis between Marxism-Leninism, ecological theory, and the Wisdom of the Deep Ones Who Herald the Awakening of Yog Shibburath Jr the Eighth, He Who Shall End Global Warming and Usher in a New Age of Ecological Stability and Peace, He Whose Tentacles Shall Choke the Life from Capitalism, Whose Beak Shall Devour Fascism, Whose Ink Shall Paint the Land Red with the Banner of Communism and in So Doing Liberate All from Impending Ecological Collapse!