So let's have the final word on this right now: Blizzard has tons of people to kill off, and even if they started running out of people, they've created enough heroes since WoW started (an all-Saurfang instance, anyone?) that they'd be fine on lore for at least the next decade of expansion. There. Now when it is discovered (and it hasn't been discovered as of this writing yet) that we're killing Arthas off in the next expansion, no one needs to raise a fit. Besides, I kind of like that Blizzard is starting to put out some hits on heroes we know and love. I'd rather mix it up with famous Warcraft faces than kick around some nobodies just so the almightly Lore gets protected.
But I do have to agree with Vorith's other point: it might be kind of nice to do something with these NPCs besides kill them. I think he forgets that Maiev played a nice role in the Illidan fight, and that we've already been told we're going to interact with Arthas a lot outside of the actual raid, but yeah, the raid boss killing us might make for a nice change.