I believe raiding is the single most important aspect of WoW. Sure vanilla lured a lot people in cause of the franchise's strengh and the fact that MMOs were new for most players. But raiding is what kept WoW going strong in the following years. And then they lost a lot of people cause of content droughts.
If they abandoned everything except raiding and dungeons there wouldn't be any content draughts. Stop making a new content every addon and leveling zones that is just not worth the time and effort. The questing experience is a chore for most people and done much better in any singleplayer rpg (mass effect, dragon age, witcher, kotor, etc.).
Emphasize the multiplayer aspects of this game. Multiplayer content is the future (see Mobas), very mediocre fake singleplayer content (kill 20 boars, collect 30 teeth) is the past and won't be tolerated any longer. Group content (primarily dungeons and raiding) is what made this game so successful.
Make WoW great again!