IMO fantasy and science fiction are similar in that they are literary genres in which the author alters the mechanics (or "physics") of the world in which the story unfolds and the characters interact. In fantasy, how the mechanics work is relatively unconstrained, with the exception that, usually, good fantasy makes sure these new 'fantastical' rules are consistent from beginning to end.
With the advance of science, and the realization that the natural world worked according to a series of rules (i.e., physical laws) that were both objective and invariant, it became possible to write stories in which the mechanics of the world were altered, not fantastically, but by using different physical rules and laws. You could either disregard existing laws entirely and use new ones on the basis of being in a different time or location, or by speculating about what the laws would be like in an area where real science had not provided answers.
A science fiction story is a story where the setting of the story unfolds not according to the established understanding of science at the time the author wrote it, but by altered scientific rules. For example, according to our understanding of physics since 1905, faster than light travel is impossible. Therefore, a story where the characters are traveling faster than light is science fiction. If the author is (relatively) lazy and does not explain the faux-science which allows this to happen, it's more like soft sci-fi, while a story that allows FTL travel by means of exotic ideas such as wormholes, or Alcubierre drives, and attempts to explain these FTL drives and how they work, is more a hard sci-fi story.
When one writes a story that takes place in the future (or past) but uses the same physics, whether that story is sci-fi or not is a difficult question. If it's in the past, I would have to say no, but a future story I would say yes, because almost by definition one usually invents at minimum new technologies as part of the story, because it is a historical fact that technologies tend to evolve over time. Strictly speaking, I prefer to use sci-fi only if the scientific laws and understandings in the story are different from reality, not just technology, but it's not a hard and fast rule.
How does this relate to the topic of whether the sci-fi genre is exhausted? I don't think it's a problem of being able to comprehend the future, because creating a distinct setting for a sci-fi story that takes place in the future is the easy part, one that any competent sci-fi author can do; all it requires is a basic knowledge of physics, being clear about what laws you're altering, and being rigorous and consistent about the consequences of those changes.
The hypothesis that technological evolution is such that it is altering society at a pace beyond which writers can cope is better, but it's still beside the point: the point of a sci-fi story is not to be both a precise prediction of the future and a precise explanation of the present. The point is to create
future which helps to illuminate the author's view of the present. There's nothing that says that either of these views have to be correct, just plausible
If authors are not careful about working within this fundamental constraint of sci-fi, then of course they risk creating mixed-genre stories that are neither science fiction nor fantasy. That doesn't mean the stories won't be good, but it does mean you can lose the strengths of each genre without compensating for their weaknesses. And stories which are based on fantastical technologies, that are not carefully thought out as to their consequences, can fall prey to what I call the Jetson Fallacy
: the idea that you can have radical technological change without radical social change that impacts your characters.
The observation that 'there's nothing new under the sun' is very old, so the burden is on current and future sci-fi authors to either recombine what's old to make it appear new, or to truly create the new. They should welcome the challenge; after all, successful science pushes the boundaries of actual knowledge. Compared to that, pushing the boundaries of fiction should be child's play!