Maybe, or gaming (And gamers' expectations of it) has changed in the past 17 years, and with the number of other online games around right now, the number of people who want to plan around gaming has diminished. (How many people play any MMORPG at all right now? I think it's fewer than played WoW around the time of WotLK)
Also, WoW subscriber-wise isn't in a position to tell a (Presumably large, considering they're catering to them) part of the playerbase "Ok, you just naff off, it's 'group up or get out' now!"
This isn't really my point, WoW's strength itself was always that you could enjoy the largest parts with solo play as that was way more convenient compared to many mmos before where you sometimes had to schedule with your friends just to level, they absolutely should continue to support and bring in more features where you can just jump in and play.
But strictly talking about the endgame reward loop/structure the best things at large have always come from group content because it makes sense to give players an incentive/excuse to break the inconvenience and build/play with a group/community, undermining that wouldn't be a good idea and probably isn't something the casual playerbase wants anyways.