I think it has less to do with the departure of individual writers (let's face it, those were always pretty weak) and more with their general approach to creating lore these days. Classic was great to me because it simply created a world with a few interesting characters and smaller stories to draw you in but beyond that there were no big heroes and there was no "narrative".
The more WoW is focused on narratives the worse it gets. The team should be preoccupied with creating an interesting world for us to explore and write our own stories in instead of spending 5 content patches detailing Sylvanas' mental health crisis.