Yeah, right, I didn't play much WoW... just from day one. I can remember the days when we wanted to open the gates of An'Quiraj (spelling escapes me, to lazy to look it up) and had to find a secret middle-of-the-night time to get it done because the server could not bear all the spectators at 19:00, 20:00 and 21:00, crashing each time.
You have thousands of players in your viewfield? When did that happen, yesterday? I stopped playing 3 weeks ago but your graphics hardware couldn't cope with 1000 players being rendered with the WoW engine back then and I bet it cannot today. My server/connection started lagging when a few hundred people were in the same zone (Dalaran comes to mind) and not even in view.
And oh... look
here someone was banned for crashing a server just this year. The ban was taken back because it was not his intend, but anyway... lots of players in one zone doing stuff is way more than a server can handle.
Edit: and yes, you can throw money at it. But that's just a waste of resources when normally, players don't group up that way. It's better to have the server crash once in a blue moon than spending another bazillion on cluster costs. It's the same logic by which we only own a ~1000€ gaming machine, not a 25K graphics cluster. Because 99% of the time, the gaming machine is more than enough.