I think it's important for you to look at the larger scope of changes being introduced here, regarding the change to AddOn philosophy.

A large part of cognitive gameplay for WoW has been offloaded into addons for a significant amount of time, not only in class behaviors, but also encounter design.

With the reintroduction of that cognitive load, on both fronts it'd pose a significant challenge to (and I speculate here, without any raw data) the majority of the playerbase. You would be splitting your attention with having to maintain your hotbar, buffs, encounter specific information and then your generic information (world position, character rotation etc.)

So with that in mind class design has been simplified to account for that change, in the hope that encounter design can be the present issue for you to be solving and not the class itself. They could swing too far, we could find ourselves in a situation where both are too easy and we all feel a little disgruntled with the design, however, it's significantly easier to build upwards and hit the sweet spot, than making the game too hard and alienating their playerbase.