Definition of Science Fiction
It seems to me one difficulty is caused by trying to describe the genre of science fiction. It's about as useful as labeling nonfiction as a genre, then trying to describe it's unique characteristics.
Science fiction isn't a genre in the way that a Western or a mystery or a romance is a genre. The reader picks up those kinds of stories or novels expecting certain themes and tropes, a certain kind of emotional payoff.If you pick up a science fiction novel, you don't know what to expect. That was true when Wells and Verne were writing and it hasn't changed since.
Science fiction is infamous for bleeding into other genres. For example, thrillers can be set in the near future, with plots that hinge on extrapolated technology. There's no reasonable way these can't be called SF, yet such a thriller still belongs in the thriller genre. If you try to label a blend, you soon discover that most science fiction is a blend. That leaves the unanswerable question of why you'd bother to talk about science fiction as a separate genre in the first place.
For example, Kage Baker might write about time travel, a fantastic element that is tradionally part of science fiction, and therefore doesn't even need to be explained in detail. But the genre she is writing is romance (at least, the Kage Baker novels I've read are romances albeit in a science fictional mode.) A fan of space opera, which is a genre, will not generally like a romance, which is a different genre. I mention Ms Baker as a counterexample to the thesis that science fiction is a subgenre of boys' adventure stories.
A novel like Allegra Goodman's Intuition is much more scientifically based than any space opera, even Alastair Reynold's. Yet fans of the space opera, which really is a genre, might be actively repelled by her novel. Again, there no reasonable way that a novel about the scientific process can't be regarded as in some sense science fiction. Yet it is so little removed from reality that the publisher never dreamed of marketing it as science fiction. Nonetheless, it's science is still fiction. It also is in no sense an identifiable genre novel. Well, unless you somehow call serious or art novels a genre. But using any words with such elasticity doesn't seem kosher.
In another example, the TV series Jericho was set in an imaginary future after a limited nuclear assault of some sort on the US. This is obviously fantastic, in the indisputable sense that it is a product of the imagination, not description. It would be deranged to call it realistic. The post-holocaust story has been common enough to be tagged as a subgenre of science fiction. Yet people have still argued that Jericho was not science fiction!
Science fiction and fantasy are marketed separately for the good reason that not everyone likes the fantastic in fiction. Indeed, not everyone likes fiction of any sort. The importance of the fantastic element in the science fiction mode is revealed in the difficulty some people had accepting Jericho as science fiction.
The notion that science fiction is therefore a branch of fantasy does suffer from stylistic insensitivity. In discussion literature, even popular literature, this is an insuperable difficulty. A fantastic element that is explained as somehow natural (especially if the explanation really is a plausible speculation!) just doesn't have the same style as a fantastic element that is frankly supernatural. Kage Baker and Sherrilyn Kenyon may both be writing romances but one has an entirely different flavor than the other. If the store's big enough, they will not be on the same shelf. But they will not be in the same place of realistic (superficially, anyhow) romances, either.
Many people live daily lives in a materialistic universe of natural causes and effects. At the end, they hope God or the afterlife will suddenly appear. There are similarly inconsistent science fiction stories where God or the afterlife suddenly appear. They don't tend to be very good, but they are not fantasies in the same way as when angels are part of the plot from the beginning. If supernatural ideas are dressed up in natural form, as in nineteenth century spiritualism, with its ether and protoplasm and vibrations, or as twentieth century crank psychotherapy, as in Scientology, well, that's an old tradition. The point is,in literature, style is essential, not an irrelevancy. Thinking so is philistine. Mixing science fiction and fantasy doesn't work well generally, as witness C.S. Lewis Space Trilogy.
Another difficulty in defining science fiction is trying to encapsulate its unique artistic contribution. Most literature is popular literature and has no great artistic contribution to make. As a science fiction fan, I too have been tempted to find a definition of science fictions that rules out everything but the good stuff. Doesn't work, I think.
I would like to think that science fiction, as fiction informed by a scientific worldview or fiction concerned about the future, has some intrinsic merit. But upon reflection, how could such noble things be excluded from ordinary serious, artistically ambitious literature? In practice, it is obvious it is. But I don't think that means science fiction is really different, or even really separate. It means there's something narrowminded about current elite definitions of literature.
Living memory shows that the world has changed. Thinking it won't change more is mad. Writing as if it won't is bad.
But failures of the would be great novelists and short story writers are not our successes. Looking at bookstore shelves, it is plain that being a science fiction fan just means you have a taste for fantastic stories that still have some illusion of grounding in reality. And being a fantasy fan means you like the fantastic freed from the surly bonds of earth. If you want to claim science fiction that describes parts of the human condition regular literature averts its gaze from, you should probably start naming names.