There are lots of reasons the subs could be dropping.... Getting bored of an almost 10 year old game is probably quite high up the list. The teenagers and students who started playing this game in vanilla are now hitting their 30s with full time jobs and possibly even families.. Priorities change.
An old game with what are fairly dated graphics (I personally like the graphics style) isn't going to attract new players in the way it did 10 years ago no matter what happens in the game.
It's pretty narrow minded to make the link that sub numbers = game quality when you are talking about a game this old.... Most games don't even keep a decent sized player base for more than 1 - 2 years old so the fact this game has around 7 million players after all these years probably means the developers have done a very good job of making the right changes to the game.