The root of all evil in gaming is horse armor. Cosmetic DLC paved the way for microtransactions of all kinds.
When players know that the best rewards are received for spending $5 rather than performing in-game triumphs, the most important core drive to play is invalidated.
At this point WoW even treats its subscription as a microtransaction. They design patches where logging in is all it takes to invalidate and replace your equipment, and further progress is primarily a function of weeks played paid.
To be fair what alternatives are there? You can't make tactically/strategically difficult content with video guides of exactly how to play. So that leaves content that is difficult because it's twitchy, which cuts off most of your userbase and exacerbates any lag in the game, content that is grindy, which is just boring shit, and just giving everyone everything on day one, which means nobody has a reason to keep playing. And to be fair, most games, WoW included, have all these other types of content as well.
Most people just don't want fancy gear badly enough to spend 5 days pissing in a bottle while farming crafting materials when they can get equivalent shit by playing 30 minutes a day for 5 weeks.