And not just Humans, but Federation-ers of all species. Voyager gets stranded in the Delta Quadrant, some crewmembers die when they are pulled there. They pick up Neelix and Kes. Torres is holding things together for a little while, but a few more crewmembers die in an engineering accident, and she's forced to admit that Voyager's state-of-the-art systems are a little beyond her - so Voyager puts in at an intergalactic trading outpost, and they put out an ad and hire an engineer. Then they rescue a few refugees from a damaged ship, who stay on board. Then an away team gets completely wiped out by Hirogen. Another Delta Quadrant native has a half-working cloaking device that he is willing to trade to them for helping him rescue a relative being held by a totalitarian government. They stay on board. A couple of security people get killed in the rescue. Eventually, Janeway begins growing less and less sure that there is even a point in trying so hard to get home, when more than half of her crew isn't even from the Alpha Quadrant anymore, anyway. She presses on, but doubts grow...
The death trope is very easy to do, it's the weakest way to motivate fictional characters, and it grows old and tedious after a while. I wouldn't want to watch a show like Trek and have to be afraid my favorite character will bite the dust sooner or later. The characters are the show, not the ship or the technology or even the ideas, the characters are ultimately what draws people in, you can't discount them or use them like disposable props.
Well, we wouldn't have to kill off the main cast unless they were leaving the show, ala Kes. Voyager: where anyone that doesn't visit the bridge on a regular basis is a redshirt.
TV execs love this strategy now. It allows them to fire expensive actors before their contracts expire, even on successful shows (See walking dead). It also allows them to fire writers more easily as they don't have to care about continuity or character development. Inevitably, all shows that do this end up sucking because the politics of hollywood end up gutting the creative content in the name of the almighty bottom line.
I think there's plenty of room for shows where characters are constantly killed off. There's definitely a line to walk, between killing enough characters that they don't appear to have a 'Magic bubble of protection', but not killing so many characters that they appear disposable. Shows like Battlestar Galactica and Lost walked the line by mostly killing annoying characters you didn't mind losing too much. Shows like 24 went so overboard with it that you stopped giving a crap. (And Game of Thrones only gets away with it because the deaths tend to credibly derive from the story) I definitely think Voyager should have gotten closer to the psychological brink than they ever did, and should have had more cases where they were barely scraping by on resources. But for killing off more characters to work, they would have had to have a larger secondary cast. What happened in Voyager you got the worst of both extremes. Primary characters had the magic bubble of protection and secondary characters seemed like useless fodder.
Getting rid of pretty pixies on VOYAGER was a very bad idea! Particularly the beautiful, guileless and vulnerable Kes! Not picking up Rain Robinson as a regular crew member was another bad idea! Making Chakotay look like less of a man for pining after the Captain for years, like a love-sick puppy, instead of taking her, like a Proud Warrior ... BAD IDEA!!! And guess what else is a bad idea?! That's right ... killing off main characters in this blessed series! But having enemies who could be considered more of a "real" threat would've helped add to the drama and excitement, that's for sure!
Killing or removing a main character from a show isn't always a a bad idea. Sometimes for whatever reason the character isn't working out, isn't well recieved, etc. Sure there might be some grumbles when a less popular character is removed, but remove a very popular character well we've seen examples of what happens then in other shows. i.e SG:A when they killed off Dr. Beckett the fans made their feelings known and they ended up having to bring the character back (but then again perhaps the way they killed him off in the first place exploding tumors didn't help)
I liked Beckett's death. I was totally unspoiled for it and it made me cry. His return shocked me. The whole kill 'em, return 'em cycle was an all round hit for me with that character So in Trek we have.. TOS: Well that Romulan commander in Balance of Terror guts me every time but of course the answer is, no one. TNG: Tasha. Meh. Movies: Spock, Data, Kirk. STILL MAD ABOUT KIRK!!! DS9: Jadzia. That had impact and was used well, though never a big Jadzia fan her marriage to Worf made it quite a sad death for me. And of course adorableness followed. ENT: No one died in this show other than baby Elizabeth in the finale, which was sad of course but we all know that Trip and T'Pol might just go on to have another, grown at home baby together. Yeah VOY should should have offed someone. Tuvok would have been a tear jerker, both for the Captain and because we know he has family back home.
Killing off characters should happen because it's the natural result of the action of the story and for no other reason. It's fantasy breaking when it feels like a character was randomly tossed in front of a bus just because the character became inconvenient or to add shock value. You start to believe the fantasy less. I like to call that "Plot lightning". But of course it goes the other way too, you also believe the fantasy less when characters survive by magic just because they're main characters.