If the game doesn't encourage longbows, why would you assume people signing up to play space cowboys will choose longbows?Well I am looking at this from a trad or classic point of view. In such a game, concrete differences between weapons tend to matter more. A sci-fi version of 5e that doesn't discourage longbows will see PCs with longbows. In a narrative/storygame, genre emulation might be adhered to without a mechanical difference.
If they all pick the d8 longbow over the d8 laser rifle, they didn't want to play space cowboys in the first place.