As for Arthas and Illidan getting killed "easily", I would have to say that I disagree. Illidan was killed by Maiev and Akama, who were aided by a band of adventurers. Beside, don't you think that making Illidan survive yet again would make his entire character pathetic? Imprisoned after War of the Ancients, banished after getting freed, failing to destroy the frozen throne, fleeing to Outland, getting caught by Kil'jaeden and then falling to Arthas at Icecrown. Aside from killing Tichondrious, he really didn't accomplish that much. Having him beaten but not killed again would more or less make him the "Epic Fail Guy" of Warcraft.
Now, as for Arthas, while we don't know how he will fall, I think it's safe to say that it will involve The Ashbringer, the only weapon which might stand a chance against Frostmourne. While we do not yet know the details, I think it's pretty safe to say that he won't go down "easily" seen from a lore point of view. And as much as I love Arthas as a character, I'd say that his defeat is necessary for story development.
Is anything sacred to game developers?
Why should anything be sacred? Why should developers cling on to one concept or character and never allow it to disappear? Truth be told I'm pretty happy the Warcraft characters don't live forever, it allows the story to progress, otherwise we'd have the armies of Azeroth bouncing back and forth between Northrend time and time again without really accomplishing everything. Beside, for every major villain that falls, a new one is introduced or one that has been dormant returns, like "The Nightmare" and Deathwing, so there's no need to worry that the Warcraft lore will die just because a few of the current characters die.