I used to think it was the other way around, casters have always been strong earlier as they scale better with abilities then melee classes do, and late expansion melee hits like trucks considering they scale with weapon damage. Wich in the end means the higher item level your weapon has the harder you will hit.
If they could find a way to even it out so all classes was at "equal" strength I would be impressed. People seem to talk about this topic every new expansion like its the easiest thing in the world to fix. Sadly its probably the hardest thing to get fixed correctly.
They fix one thing, it makes another class become strong. See the pattern I am talking about?
This doesnt just count for WoW specifically either. In games like League of Legends this is also the case, they nerf 1 champion another champion becomes broken since the game needed that specific pre-nerf champion in order to keep the new OP at bay.
Believe me guys if it was just for blizzard to push one button to even everything out they would, it would save them so much time and they could spend their recourses elsewhere.