That has nothing to do with the engine (which has been updated and overhauled so often throughout the games life that it is already functionally running on a more modern engine than it started with). WoW has had all of those mechanics at different points and each time the community at large hated them. Its not Blizzard refusing to do it or being held back by the engine. They are listening to the majority of their playerbase, who overwhelmingly said they don't want those things as a major aspect of their gameplay.