If you have a look at how player numbers have declined for any other mmo, their numbers are amazing. Can't forget that WoW come out in 2004, the life span of most games is measured in months.
The game doesn't hold people's interest indefinitely, even of it is a lot longer than most games and probably most people who would want to try the game already have. Also competition is way higher than it was in 2004.
I imagine the implied turn of phrase was actually "struggled to make the numbers". As TheCapn pointed out, an unaccounted for 20% drop in plans is bound to shake things up a little.
Well, you can only play a game for so long before getting tired of it. Sure, it's a huge world. But every world dies at one point, and WoW is no different.
they will come back eventually. the last expansion came out in late 2010 and the next is due this fall(not exactly sure). thats the way wow works. people get fed up afeter some time without enw content and then rush back when theres something new.