It was a slow death, one by a thousand papercuts, you can see the changes in Wow as an example.

Classic Wow and BC were not convenient, to enter a dungeon you had to travel to it, often you had to finish the quests in an area to unlock it, if someone wanted to come who hadn't done this yet you were compelled to help them, often these were quests that were beyond what one person could do. It was built around teamwork and collaboration, but at the cost of convenience.

Starting with WoTLK the story got better, animation and art got better but also a lot of odd convenience things started making their way into the game, and often at the expense of community. Most people at the time realized this but it wasn't strong enough at the time to erode the other things they liked. This was right after Activision merged/bought them and I would say they still had 80-90% of their core staff then.

After that was Cataclysm, which most players do not look fondly on. Though I would say that again they did well with storytelling and Art, but made poor gameplay choices. Most long time players quit playing then. Some really important leadership left during this time or right before it was released, often due to conflicting opinions about direction. I want to say microtransactions for vanity items were introduced at this time.

Then it was Mists of Pandaria, which I think they restructured their leadership quite a bit and put some of the right people in charge again. I think they were down to 60% of their core staff by this point. This one was mostly considered a success and is looked on as mostly good. It was around this point that Blizzard got really involved with the Chinese market or at least more involved. I want to say this was when they introduced services like buying a level boost and race change.

Warlord of Draenor - most players see this as one that had a ton of potential but never really grasped it, most new features were considered not fun, even more convenience things appeared again lowering camaraderie and need to work together. Eventually it was abandoned halfway, the story never finished and so it was scrapped for the next one that they hoped would be a more compelling theme/setting. The art got very good though, but it felt like we were getting less bang for the buck somehow. It was around this point that the game shifted focus to just help everyone level up to endgame content, making the old content barely matter anymore, which is a shame in a lot of ways.

Legion - was mostly considered good, they implemented an artifact system that acted a lot like a re-balance patch on the mechanics of play, this was where most of the praise came from. But even with that there were a lot of complaints, world quests felt like a chore, and so when people developed ways to get through it quickly they patched it to slow it down. The region Suramar got a lot of praise since it was a region that would change based on what quests you've done and I think going forward they should learn from that, but so far it hasn't felt that they've learned much.

Battle for Azeroth - This was a failure of an expansion, people didn't like the characters, the story direction or any of the new features introduced, the game balance was poor and the new artifact system didn't act like a patch like the previous expansion and instead was just unbalanced. It feels like they skipped to the next one fairly quicly. Probably 25% of core staff still remains at this point.

Bonus - Warcraf 3 Reforged - for those who are warcraft fans they like the lore, many grew up with the RTS and they were looking forward to this game. Blizzard failed to recognize the importance of branding here and instead looked at RTS as a dying genre. They overpromised, got pre-orders and then when it was time to bump it back to keep those promises they instead opted to not refund pre-orders and instead release a known inferior product that was different that what was promised and in fact different from what was pre-ordered. Honestly I wouldn't be shocked if more lawsuits came because of it. This was a final nail as a breach of trust for many fans who have felt continuously betrayed up to this point.

Shadowlands - it had good ideas but never lived up to any of them, people came back to try it out for a few months but it's mostly abandoned by players now. probably 10% of core staff still remains, it's a sinking ship.

So yeah it's been a failure 13 years in the making, that's just been exploding all at once the last couple of years. Blizzard has failed at branding, especially since joining activision. Blizzard used to be a company that made high quality computer games with an ethos of "it's done when it's done" and would go so far as to not release a game (starcraft ghost) because it wasn't good enough. That's not who they are anymore, they release things that aren't finished, that are outside of expectations or that are at a disconnect between what their fans want(like game balance over flashy story, community over convenience, or feeling like a part of the world instead of you being the center of it). I think they have some great developers who get it and have advised management many times about this kind of stuff, but a decade of not being listened to has made those with any clout to go off to other companies, often founding their own.