They are shaping the game to suit MMORPG players better.
I wouldn't be surprised if we get Alexstrasza, Elune and Kil'jaeden as killable bosses soon enough.
LOL why u wanna kill friendly bosses? Elune is kinda night elf godness, alexstrasza was helping night elfs on first war, and kil'jaeden.... well is as powerfull as archimonde so he is weaker than arthas...