I don't think there's anything in the game justifying voice actors. Starsector isn't plot driven in the slightest.
Well, to be fair I think that is kind of a "for now" situation. Plot-driven content would by necessity have to be last because you can't change a plot willy nilly and it has to be based off features and mechanics that are not yet finalized. Alex has hinted before that he wants to do more lore/story content eventually. That doesn't necessary mean plot, per say, but it very well could.
Yeah, the problem with voice acting is that it is all or nothing. There isn't much in-between, and it adds a burden on all future content, as well as making mods stand out in a bad way since most will be lacking those.
Definitely. Voice acting can also end up as a detriment to a game rather than a selling point. Bad voice acting < no voice acting. I loved final fantasy 7 and 9 and those games' insanely rich stories would have been ruined by voice acting. On the other hand, final fantasy 10 had it across much of the game and Titus' voice still makes me grind my teeth at times lol.
There is something to be said for leaving a character's voice to the personal imagination. Book adaptations into movies highlight this well. Sometimes I see the move and think "that is not at all how that character sounded in my head" and it can sometimes even be disappointing enough to reduce the quality of the movie in my eyes... though maybe I'm just a snob in that regard.