I don't think sf / fantasy / horror genres are strongly related to "speculative fiction". Speculative means "based on conjecture", and conjecture means "formed

I don't think sf / fantasy / horror genres are strongly related to "speculative fiction". Speculative means "based on conjecture", and conjecture means "formed on the basis of incomplete information". Incomplete information isn't what I think is at the heart of fantasy, and usually isn't what I think is at the heart of SF either. SF is usually a story about an idea rather than a character; it's not speculation. Fantasy is usually a heightened, exaggerated world - a world with the saturation and contrast turned right up; again, not speculation. Speculative fiction I'd place in a niche area of hard SF that is more concerned with futurism than ideas per se. That is, it's interested in predicting how today turns out with some reasonable technological advance and specifically how the world deals with that advance, whether on a global level or via a character study. As to needlessly multiplying entities, I agree, with caveats. It's related to Chekhov's gun: don't introduce elements that don't matter (and similarly, if you rely on something later on, ensure it's foreshadowed rather than parachuted in as a deus ex machina). OTOH, a close adherence to Chekhov's gun can make stories very predictable, for people who pay lots of attention to plots. And other people come for prose style, and still others for world-building; neither of which are overly concerned with omitting details.