So far I played 5 mmorpg games and all suffered from this issue and lost player base/went down.
Answer is simple:
With patches you buff characters, while monsters remain the same. Once charming game where players have to struggle and cooperate becomes one man slaughter. While challenge goes down, satisfaction from completing adventure also goes down, with result of players becoming bored, game not rewarding, and they leave.
Simple psychology rule: more challenge and cooperation = more satisfaction. Just keep the balance edge -too difficult may frustrate
What is funny, to battle player loss developers buff characters even more in hope of luring players back to game. You can guess the result. Also the more buffed character is the less buffs it needs, making offers from devs less attractive or just useless- you will kill everything anyways.
Developers these days forget what game supposed to be, and they fail to keep balance of difficulty making games walk in the park where players switch games frequently because nothing hooks them up.
This is your answer. Might be too late tho..