There's always the danger of reductivism for the sake of reductivism, though. Georges Polti argued there were only 36 dramatic situations; Aristotle broke it down even further than that. Also keep in mind: the basis of this discussion is the argument that "Star Wars" isn't sci fi. That right there pretty much makes the entire discussion esoteric at best. I think it's important to distinguish between "does the 'space' element matter to the story" and "can the 'space' element be replaced and still have a story." "High Noon in space" is still in space; it's a different movie than "high noon." "Samurai movie in space" is still in space. And, as discussed previously, Alien was originally set on a B-24 liberator in WWII but the movie as made does not happen on a B-24. It's not like any of this is binding anyway... but I'm cautious about defining away anything that anybody normal would consider a "space" movie. Eliminating Moon, Event Horizon and Silent Running does exactly that.
Oh we are definitely off the beaten track in terms of typical definitions, I personally wouldn't make the assertion that Star Wars is not a sci-fi flick or that Gravity is not a space movie outside of this post. My point was that the space element within the movies I listed played (to different degrees) more as a backdrop to the story rather than being a fundamental irreplaceable part.