WoW is on tier 14 not 15. Anyway it works keeps people playing. But now they have 3 difficulity levels, LFR (easy), normal, heroic (hard) for their raids. The system of gear progresson is proven to work and keeps people comming back trying to get the newest and latest gear. Without any gear progression or anything people run out of stuff to do really fast. Their 6 months for new tier of raids and gear is about right, its when people start to get bored of running the same set of raids week after week.

People keep saying WoW is easy but its not really, your average person can beat normal modes after they nerf it perhaps, but the hard raids heroics only have about 5% of the player base actually finishing them