One thing you need to realize otaXephon, the way vanilla wow was structured makes it very hard to have a content drought. Yes, at the point you have a full set of BiS naxx gear, an epic mount, maxed professions, grand marshal/high warlord title, yes, you've beaten the game. Vanilla wow didn't let you skip content like that though. It made you walk through each step of its progression to see the next tier of content. So while there would be an end (which I personally find attractive), it would be such a long journey to get there that it would be well worth it.
(For those who didn't get the point, you raid MC until you get into BWL. BWL to AQ40. AQ40 to Naxx. This progression line ignores all leveling, dungeons, professions, and smaller raids required to complete that content. There is always something new around the corner... for a very long time)