Which is why the reveal of the Expansion already has to be big. Almost in the way of "back to the roots" of some sorts, with a new Class (or even two, who knows?) and more to do that is fun, but not forced.
Its pretty much a 50/50. If the Expansion reveal itself already looks to be barebones, good night World of Warcraft, 20 years have been enough. When even the Sheep leave the Game you know you goofed.
But they can still turn this around. They have the money, they have the time, they perhaps have the people. It does not take a genius to look back to the previous Expansions and do what worked regards Classes and Content. You just have to play Video Games.