It is in no way this black and white, it is no vacuum.
Most casual players (who carry this game financially for sure) are not anti social, but they can't or don't want to invest the time and their mind to this game so much that they seek out guilds/groups, i was like that for most of the early expansions until i found a classmate who raided starting in mop.
WoW was always build around these players day one, you could always quest alone, grouping up was more useful/convenient back then but never forced to play the game, for months if you played casually you could just level and enjoy the game, do whatever you want (just like now too btw )
Also i really disagree with the more hardcore than ever part, you don't understand the mindset of most casual players i guess, you really think most players care about doing daily stuff and getting everything done they could? hell no.
Edit: if you want a representation about how committed most players are: look at views from youtube guides/in-depth game discussions and then look at ranty stuff and or WoW related memes.
Same with the WoW reddit even though they occasionally get more into meaty stuff like now with the rwf, usually you'll find a ton of memes and artwork there but not much critical thinking about gameplay aspects.
Hell, look at what NA general discussion is like..