I think there's a striking correlation between the merger of Vivendi and Activision and Blizzard's steady decline as a publisher. Since the merger, we've seen Blizzard:
Dumb down World of Warcraft in an attempt to reach a broader audience, to the point where the game no longer respects the player's intelligence. To quote a post Ghostcrawler made after he left Blizzard to join Riot Games, "it's really refreshing to work on a game where I don't have to worry whether someone's grandmother can pick it up or not."
Fill World of Warcraft to the brim with MTX. Cosmetic changes, faction transfer, realm transfers freely between PvE and PvE realms, character level boosts, pets, mounts, tokens which allow you to legally buy gold for real world cash, cosmetic armor sets, etc.
Gradually decline the amount of World of Warcraft expansion content since Cataclysm. Legion is a bit of an exception to that rule, but that's probably because Blizzard realised that a 15 month content drought during an expansion with only 3 raids and 7 dungeons almost killed their game outright and sparked a mass exodus towards Vanilla WoW private servers.
Sue the Korean eSports Association (KeSPA) over broadcasting rights for StarCraft: Brood War because they weren't getting enough in royalties. Blizzard pre-Activision were strongly supportive of the Korean scene in comparison and didn't try to chase royalties down from KeSPA. This inevitably did more to harm the StarCraft competitive scene.
Remove LAN support from StarCraft II so that it can keep total control over the game's broadcasting rights. This is something that Blizzard 15 years ago would never have considered. It caused major damage to the Korean scene on launch because KeSPA had no involvement in the first two years of SC2 as a competitive game, and did even more catastrophic damage to the North American scene in 2013 when the Battle.net World Championship Series was expanded, monopolised the scene with a hefty broadcasting embargo, and arguably drove the North American Star League (NASL) to bankruptcy.
Add online-only DRM to Diablo III and implement a real money auction house where players can buy and sell in-game items for real world money.
Throw out relentless Hearthstone expansions and expansions on a regular basis to the point where it would take sinking hundreds of dollars in booster packs or months upon months of grinding to build a remotely competitive deck.