If I’m being honest, the seeds for competitive gameplay in WoW were sown during the game’s inception. Keeping in mind that many of game’s original developers were from top-end raiding guilds in Everquest.
The thing is… WoW was a breakout hit for the genre, attracting far more people who were NOT into the competitive aspects of the game. But I think it became more obviously competitive once the game had peaked and started to decline in population; so around mid-WotLK.
Non-competitive players tried the game, had their fill, and left. A select few stuck around for the handful of good times they had, but the game is pretty much an endless uphill struggle for them because there are some fundamental disconnects with the game’s design and how they want to approach it.
Those who say WoW is “more casual than ever”? They can’t really see the forest from the trees here, as it seems the game is still built around the mindset that the player wants to focus on progression in some way. So there’s avenues for easy progression, but it’s not what the “casuals” actually want from the game.
The nearly-unavoidable counterpoint in FFXIV, which is designed with a fundamentally different approach that is much more narrative-focused; it’s quite obviously modeled after the single-player JRPGs of old, which puts the narrative front & centre. This is much more casual-friendly and closer to what many consider an RPG.
Conversely, WoW’s based off another RPG - Diablo, which is the premier example of running the same thing over and over again for a chance at some randomized loot that might be an upgrade. Storytelling had never really been Blizz’ strong suit, aside from cool-looking set-piece moments, and that has only become more obvious over time. And let’s be honest here, WoW is only JUST starting to get past the “bad fanfic” levels of writing… and even then, there’s nowhere enough time being given for the narrative to play out properly.
Anyhow, the point here is that a good story carries along casual players far better than gameplay systems… and it comes down to the simple fact that casual players don’t really plan on sticking around through the game’s life-cycle. A good story for them means they’ll have a good time for the time they’re playing the game, while systems to keep them sticking around become a source of frustration as they have the stuff they’re interested in blocked by arbitrary grinds.
Anyhow, WoW’s design has pushed a lot of “casual” players out… which made competitive-minded players more prevalent, turning into something of a vicious cycle that further reinforced that competitive nature which keeps driving out non-competitive players.