500 would be a fantastic deal, if it included ever Xpac as well ;O But i could see it happening in a way as a free to play, but not free sort of thing. They know full well that players invested in the game like that would still be paying for the other things, boosts, pets, and such.
More likely to not include Xpacs if they ever did something like that.
And not every player would go for it anyway, So it may be a good idea based on what they are seeing that we are not.
Or it could be a really bad one based on that.
But i suspect WoW to at least be around in some form for 5 years yet, So even without Xpac included. If its around that long and you play it, it would be worth it to a player over the time for some.