Despite a near religious fervor for Reese's Peanut Butter Cups, I much prefer to keep my Fantasy Peanut Butter and Sci-fi Chocolate well and truly separate.
Sci-fi is great. Fantasy is great. But I don't want magical artifacts and pegusi in my sci-fi or ray-guns and space ships in my fantasy.
Yeah, of course, I've run through Barrier Peaks. We all did in those days. But we didn't take away ray-guns or androids to further sessions.
The excellent point of when the sci-fi/fantasy genres were not so well defined as today and it was popular to mix elements of tech and sorcery, as in such things as He-man, Blackstar, Thundaar the Barbarian, et al. of the time, Thundercats and others that came later...all of which I thoroughly enjoy...doesn't really change that if I'm sitting down to a D&D game...I'm looking for spells and elves, not hover-boards and destruct-o-rays...and vice versa, sitting down to a Star Wars/Trek/Galactic-Rangers or any other "sci-fi" game/story, I'm looking for inter-stellar ships and alien races, the most mystical thing that I should see is psionic powers.
It's just what I prefer. Can the argument be made that it's all a matter of aesthetic and window dressing? Sure. That the elf with a magic wand in one game and a vulcan with a phaser in the other are exactly the same thing. A permanent levitate effect on a carved plank of wood or a broach of shielding are the exact same as a personal hover-board or forcefield generator. That interdimensional portal arch if the same as a bio-molecular transporter.
But, if I see a wizard in a fantasy world walk in the door with a laser-rifle strapped over his shoulder, I'm outta there....mentally, I mean. My immersion is just broken. The game becomes caricature in a way...or, well, more caricature than I am willing to accept since the whole of any of these games are a commonly accepted collection of tropes...caricature of itself, nowadays.
There are a certain accepted tropes and limits to disbelief around which each rpger draws a line to play, well, any game. Obviously, where these lines are is simply a matter of personal experience, preference and expectation. There is no "right answer", at most small groups of individuals might have mostly common ideas of where/how these lines are drawn.
For me, as much as I enjoy stories like He-man and Thundaar, I don't necessarily want to play in one of those worlds.
So a vote of "Absolutely not" from me. I'll enjoy them separately, thank you.