I think it is a matter of huge hubris to think one should "reinvent" a genre. I don't understand this obsession with "changing" genres. Genres exist for a reason. They are a form of shorthand that helps readers/movie goers identify the key themes and expectations of the story.
Alien is a classic sci-fi film because it asks the quintessential question of all great sci-fi: What if? It takes a rational approach to the science. Sci-fi isn't necessarily scientifically accurate per se (though hard sci-fi will certainly strive to be as accurate as possible). But it does approach science rationally and ask "what if": in this case: what happens when we finally do figure out space travel and come across lifeforms that may not actually be friendly, and how woefully unprepared we are.
Yes, it is scary. Good sci-fi often is (for different reasons). But the presence of scary elements no more makes a film "horror" than the presence of a romantic relationship between two characters in a film makes it a romance.
This obsession in certain circles to make every...single..book a "hybrid" is one of the reasons why you can't find good genre fiction with a search on Amazon. Because everyone claims they are writing sci-fi/fantasy/horror/mystery/romance crossovers and put their work in ALL those categories.
Sorry, I get twitchy on conversations about genre. I'll get off my soapbox now.
The tendency to label a book with every possible genre label is indeed unfortunate, and yes, genre labels can be useful to readers. However, there are occasionally true hybrids, or at least books that contain such large elements of two different genres that they are on the borderline somewhere.
We do allow for that a little. Put comedy and romance together, and you have romantic comedy.
Horror is a special case in that it can appear in many different guises. One of its purposes is always to provoke fear, but the way in which it provokes fear can be suggestive of other genres. Fear of vampires or other mythological creatures connects it to fantasy. Fear of aliens connects it to science fiction. Fear of serial killers connects it to mystery or thriller genres.
Sometimes stories can work successfully with combinations of different genre elements. For example,
American Horror Story is always horror, but it evokes fear in a wide range of ways. The season set in the insane asylum, for example, used a weird mix of demonic possession, alien interventions, escaped Nazi scientists, and serial killers. That sounds like a mess, but the writers got it to work. Each season also ends with a bizarre happy ending that is unusual in the horror genre.
I suddenly realized I was rambling. I guess my primary point is that I agree with you in general, but I think there is some risk of genre labeling becoming straightjacketing to writers. Sometimes, it's the people who break the rules that push us forward.
(And, for the record, I like it when you get on your soap box!)