When it comes to making fantasy and science fiction credible, there seem to be at least two approaches:
1. The “one gimme” approach: The writer introduces an element that doesn’t exist in our world and extrapolates logically from there. The writer’s implied pact with the reader is something like “Give me your willingness to believe that someone will develop a faster-than-light engine, and I’ll tell you about life when faster-than-light ships are common.” If the “one gimme” is flexible enough and the extrapolation is smart enough, the writer has lots of story possibilities.
But to keep the story grounded and credible, and to play fair with an audience that’s agreeing to accept the “one gimme,” the writer introduces as few other nonexistent things as possible.
He still has lot of room to maneuver. For instance, a society advanced enough to develop faster-than-light travel has no doubt developed other technological advances as well, so the writer can introduce those elements, too. In fact, he should.
But even those elements are logical extrapolations from his “one gimme.”
2. The “if this, why not that?” approach: The one nonexistent element (faster-than-light travel or whatever) frees the writer to introduce all kinds of other elements.
A good example is Superman. Some readers have found it unbelievable that the guy can disguise his identity just by changing clothes and putting on a pair of glasses. He may have super-strength and invulnerability and all, but his powers don’t include rendering everyone so stupid as to buy such a weak disguise.
The answer to that objection is, in essence, “You accept that a man can fly like a bird, bounce bullets off of his chest, uproot mountains, see through walls, hear sounds from another continent, and do other miraculous acts. You’re obviously willing to believe all kinds of impossible things. So why can’t you accept that the man can disguise himself with a pair of glasses?”
Of course, some “one gimme” stories are less than rigorous about never introducing other elements. And some “if this, why not that” stories maintain quite a bit of internal logic and consistency. There are pleasures in all kinds of stories.