You mention that Science Fiction was the vibrant literature of the last 50 years but that it is now dying. What made is so vibrant, and what is it now losing? And what are young writers like myself to do, if we would really rather write Fantasy-SF and not another genre?
Fantasy is not dying -- fantasy is in the best shape it has ever been in. In fact, that's one of the reasons science fiction is dying -- it has to fight for shelf space in the sci-fi/fantasy section with all those big, thick multi-volume fantasy series.
There are several reasons, I think, why science fiction numbers simply aren't what they used to be. First, science itself is not offering as many really exciting new technologies with the scope and grandeur of the new ideas available from the 1930s to the 1960s. Also, even in technologies not yet achieved, we've seen so many science fiction variants, each one so often repeated, that they feel as old hat as if they really existed. Is anybody really intrigued by yet another hyperdrive?
Second, most of the tropes were invented two generations ago and they are now so overused that many readers start to yawn just from the mention of them. Oh, time travel, right, he goes back and changes his own past, yeah ... That's why alternate history has blossomed -- at least there's plenty of history in which to make alterations, whereas we can easily feel as though we've seen all the futures.
Third, in reaction to these problems many science fiction writers have become either more technical, as they try to explain sciences and technologies that are really really hard, all the easy ones having been taken, or they have become more literary, trying to "freshen" science fiction by using techniques the writers learned from English literature classes. Both tendencies are stultifying, leading to stories that only ever-smaller audiences can stand to read.
Fourth, cyberpunk just about killed science fiction. This one vision of the future, a sort of hard-boiled-detective version of sixties drug culture, appealed to one subset of the science fiction audience but rarely offered one of the strongest driving forces of science fiction: The hero you can love, admire, care about, root for. Most of the "heroes" of cyberpunk were lowlifes that, if you saw them on the street, you'd lock your car doors and drive away as fast as you could. Such heroes have their place -- but they don't build a large audience of enthusiastic dreamers. When cyberpunk was new and all the rage, it looked like it was the salvation of sci-fi. But it's like a hot new restaurant. After the first wave of enthusiasm is over, you find out how good the cuisine really is by seeing whether it has staying power. And cyberpunk doesn't. It's an empty future.
Fifth, it's not just that the new wave of fantasy is taking up shelf-space. It's that fantasies like those of George R.R. Martin, Robin Hobb, and Robert Jordan are satisfying many of the cravings that drove science fiction -- they have the heroes, the world-changing events, and magic offers avenues of exploration that have been closed off with technology. Some of these fantasies are obviously science fictional in their handling of milieu and the underpinnings of magic -- it is arguable that Robin Hobb's brilliant series are science fiction pure and simple. So those who are reading Ship of Magic or Game of Thrones are satisfied, and therefore aren't yearning for the next Dune or the next Foundation. Indeed, you can make a good case for the idea that Ship of Magic is the Foundation and Game of Thrones is the Dune of our generation.
Sixth, many of our best new writers have been seduced by either fantasy or literary writing, and so we've lost them.
Seventh, most of us don't really care about the boundary between sci-fi and fantasy anyway, so as long as one of them is thriving, we don't much care which one. What matters is that there remain an audience looking for stories that are not the same old thing every time. I daresay most sf and fantasy readers are in fact looking for a repetition of the same book over and over again (which is why I have to hear people talk about how "Shadow of the Hegemon" isn't as good as "Ender's Shadow" when in fact it's merely not identical, and may even be better); but science fiction became the most important branch of literature for a time precisely because that's where the open-minded audience was. While the "literary" audience prided itself on being avant garde as it read endless repetitions of the same "experiments" certified by the Modernists before 1940, science fiction actually was doing new things, stretching boundaries, finding new stories to tell and new ways to tell them, all with the collaboration of a vibrant, interested audience. That audience may have moved on.