Haven't read all the posts in this thread but Blizzard have said many times (quite recently too) that they at this point see no point in making a WoW2, there is no point in re making essentially the same game just so they can improve the graphics when they can more easily improve the graphics engine every expansion.
I dont think WoW will die and Blizzard would be very stupid to finish their cash cow in favour of upgrading graphics. If WoW does get to the stage where its subs are drastically low I think their move would be to make the game f2p, give the IP a rest for a while then return with a new RTS game followed by a new mmo but only after a good break and plenty of development time.