I was thinking of Malfurion, Tyrande, Maiev, Jarod and Kerrilldank-heika actually, and those have been developed over larger periods of time as well. And while I don't disagree with that, there's still more than five and I don't even entirely agree with the five you've picked. Let alone with the idea that killing them off is the only solution. Blizzard writers may just as well use such a development as an opportunity to now develop just four characters to lessen their tremendous workload. Or they replace one character that clogged their faction's spotlight with another, like they did with Varian and Anduin. Which I'm not sure is particularly an upgrade (as an idea, not in relation to these particular characters).
And yet they pulled that off with Orcs for an entire expansion (largely about Orcs, no less). And even after Saurfang finally took up that mantle, it was mostly done as an update on the racial info on their website, without any significant echo in the lore. Killing Sylvanas isn't a guarantee that anything will happen with any other Forsaken. They can very well fester in the lore void.
There already is Kerrilldank-heika in Warcraft. It's obviously Kerrilldank-heika.
If there was a story development that made her eternal it wouldn't be plot armor... What is it with people misusing this term?