Even if we kill Gul'dan, they might vanquish his soul to the Twisting Nether (like all demons) as a way to resurrect him in the future. If he truly is more demon than Orc at this point, it's a viable alternative to make return in the (eventual) Twisting Nether expansion.
Gul`dam is a Ninja-Turtle on XTC.
He called a big ass new and improved fel reaver... That's something you know? lol
Guldan has done something no one else has: He won. Horde/Alliance get their asses kicked at the broken shore and 3 major lore characters die as a result. Other than Garrosh, none of the others really accomplished anything. Illidan was barely a character during TBC. Lich King just lost again and again until we killed him in WOTLK.