Which is a lot like life. I've always thought Whedon's intent for characters was interesting. It rarely effects how I feel about a story overall, but this time it has, to learn she was sick.
Well, technically every couple -- even the couple that's been married for 60 years -- will be broken up by the death of one of them.
But Whedon literally breaks up every couple. There's never an exploration of the drama that's involved in maintaining a long-term relationship. You see a couple in love or even "deep like" in a Whedon project, and you'd better watch out for stray bits of lumber, vengeful demons, or incurable diseases. It's basically guaranteed that the relationship will end and end badly. If not by death, then by the incompatibility of the couple that inevitably rears its ugly head.
I disagree that killing off characters is so late 90's. Like any kind of narrative story telling device it just has to be done in a manner that works for the story overall, and has impact, and forwards the story.
Whedon usually pays off his deaths well. He uses them at once for the "shock value" and for the long-term implications (when he has time to explore those). But that said, I think that, while you need the odd death here or there to up the ante, the problem is that it's no longer shocking. You know it's gonna happen. Maybe not to whom or when, but you KNOW he's gonna get lethal with some characters.
This was revolutionary in the 90s when, most of the time, a character death was a VERY rare thing on even long-running sci-fi shows, and usually was down to something like failed contract negotiations. Even then, it was more likely to be a "departure" that left an opening for the character's return for a guest spot down the road.
Whedon was a trailblazer in this respect. In fact, I'd say it's more "so 90s" to have a show where your main characters face death regularly and all come through unscathed. This is why, even though it ran well into the 2000s, I tend to view a show like Stargate SG-1 as a "90s" style scifi show.
My point is more that the shock value is gone from this particular tactic, and it's now old hat. I'd be more shocked by everyone making it through multiple Whedon seasons alive than I would be by him killing people (especially when they're in couples).
In this case, Inara, I think this part of her character lends itself to a subtle intent that Whedon built into this show. I love that Firefly still has stories to tell us, and stories that it will never tell. I don't think he did it with later shows.
Well, generally, that's because I think he usually knew the show was gonna end, so he wrapped things up nicely. Angel is the exception to this, but even that was wrapped up in comics, as I understand it (I haven't read 'em though). I enjoy Firefly but I think a lot of what folks loved about it, the comfortable, homey feeling of the show, would've been totally shattered in subsequent seasons. Certainly Serenity suggests that in classic flying-timber fashion.