Great premise, but NYer "story" stops short - Guess what, it's not really a story!

Current New Yorker "story" by David Gilbert is quite fascinating as far as it goes, which is not far enough  unfortunately. It's one of those "premise" stories: everything's going along like a perfectly normal narrative in the realist/naturalist tradition - a guy goes out for dinner and drinks with an old friend, while wife's away with the kids visiting in-laws, drinks way too much, comes home, remorseful, feels ill, vomits in the night, wakes up hung over, vows to run or put himself back together in some way - and then story takes its turn toward the supernatural: alongside the bed (where he'd vomited) is a slug like half-human creature, something like a baby or fetus - and that's where the story goes: over the weekend, he begins to care for this "infant," buying it baby formula and stuff, and he's not sure how he'll explain all this to his wife. OK, so the story such as it is works in this sense: because of the the effective conventional narrative at the outset we are primed to accept a supernatural event as within the natural parameters of the story. In other words, we're ready to believe in this possibility - maybe even more so than if we were reading a story that from the outset seemed like fantasy or horror. In those, we suspend disbelief and dive in; in this kind of story, the author earns our belief. My only problem is that - Gilbert does nothing with the premise, it just stops short. I'm guessing here - based on the NYer track record of serving as a shill for publisher spinning forthcoming novels - that this is a novel excerpt and that Gilbert works this premise much deeper in a longer form - recalling such natural/supernatural works as Rachel Ingalls's Mrs. Caliban, to one of the best. I don't mind excerpts from longer works appearing as the fiction selection in the NYer, but at least be sure they stand on their own. This one, and others such as Lahiri's excerpt in the summer fiction issue, come off more like samplers - some of the publishers used to issue groups of samplers in bound pb editions to booksellers and critics. The NYer is now doing it for them.