it's a question of circumstance, at least to me. if you have something like, say, stephen baxter's work which is [i]obviously[/i] outlandish as all hell and basically has no hope of ever coming to fruition [i]but[/i] is tonally grounded in a "future where this stuff is possible" type thing, i don't think it loses its allure at all. basically as long as it has a solid and "grounded" backing for the impossibilities within the story, and it [i]embraces[/i] them, i don't think it usually loses its allure. look at john carpenter's "the thing." it's actually a silent middle finger to thermodynamics as the titular creature is able to assimilate every cell of a body within seconds [spoilers(?): see when windows is transformed], but even though i realize that something like that could never happen, the fact that it's an alien creature means that it's something that may have an explanation, but is something that can have one. the trick, then, is not to rule out impossibilities, but to introduce them in a way that plays up the fact that we can't possibly understand the way that they've been brought to the realm of possibility within the context of the universe. call it an advanced suspension of disbelief. and then there's proper tone. like, if you brought magic into the realm of sci fi it'd look stupid, as magic is something that's simply impossible. treating it as an element of the story without contextualizing it within the grounds of reality and science would undermine the suspension of disbelief that sci fi requires. but if you take the same exact abilities that magic grants and contextualize them in a way that plays up on how science itself would react to discovering the ability it sounds fine. because as impossible as it is, it still gives off the tone that the mechanic in the story has indeed gone through a scientific process of discovery within the bounds of the story. take a look at biotics in mass effect. "what the hell is element zero? eh, who cares, it's obviously something scientific." and even then i believe that the greatest sci fi works are usually based more on philosophical conflict lying at the core of the story. jurassic park has always been my favorite piece of fiction ever created, and i can tell you that when the inevitable discovery that DNA can't be readable after 1.5 million years showed up the novel didn't lose [i]any[/i] of its appeal to me.