Let's face it; most people still around from the early days of the game are 40+ year olds who start not even gaming anymore; the younglings aren't few but they don't tend to stay especially since WoW is nothing too special in the context of new games.
An expansion that just has "dragons and Alex the hot!" is not going to cut it; it might have a small peak at launch but even that may be underwhelming; the new producers will likely pull the plug entirely and possibly try something extremely new.
The nature of "extremely new" is up in the air but I have some predictions; perhaps they will delete all technical notions of 'expansion' and release it only as 'Warcraft'; what do you think they might do?