There are definitely things that blur the line and cross genres, or things that may meet one person's definition but not another's.
I do agree it would be impossible to provide an entirely objective division that everyone would go along with.
Even so, I'd love it if all the "medieval dragon witch ghost magic spirit quest" stories could be placed on a different shelf of the bookshop to the "black hole generation ship dark forest faster than light" ones :)
The Pern novels by Anne McCaffrey feature noble warriors riding genetically-engineered telepathic fire-breathing dragons in a feudal society protecting an alien planet's human space colony from toxic spores. Which shelf do I put them on?
Perhaps the author has a voice here? McCaffrey always insisted they were science fiction. Pern was colonized by space travelers desiring a non-technological lifestyle.
"The Cyberiad" by Lem is full of "medieval dragon witch ghost magic spirit quest" stories, but most of the characters in it are robots, and they travel through space.
"Inversions" by Banks is "just" a medieval quest story with magic unless you know The Culture stories, in which case is a interstellar politics story with high tech.
So even those categorisations aren't that straightforward (I would put both in the SF category, but Inversions is tricky - someone unfamiliar with Banks could read it as a straight-up fantasy novel, and if you don't like fantasy it might feel tedious)
I'm good with a few weird edge cases. Just let me find the majority of sci fi books without having to trudge through vast numbers of definitively fantasy books!
The thing is, it's not a "few weird edge cases". But this seems like an odd "problem" to me anyway - I must admit I've never been in the situation of having to trudge through vast numbers of definitively fantasy books to find SF books anywhere...
The majority are really not that hard to categorise.
In the UK at least, fantasy and sci fi occupy the same shelving. Takes me ages pulling books out of the shelf, and immediately rejecting because they are fantasy.
The majority of the books are fantasy, not sci fi. Fantasy seems to have a much bigger audience in the UK anyway.
I'm in the UK. We must frequent different places, because I've never had that problem.
Well, it's a reasonably big place. It would be surprising if we did frequent the same places!