They failed to capitalize on custom wc3 maps, so dota allstars spawns the moba genre with LoL, Dota2, smite, and countless others, tower defense games and other maps all spin off to make their own games. They make SC2's custom map functionality worse which cripples any chance for capturing further innovation.
They completely torpedo e sports with the release of SC2 which many consider just inferior to SC:BW, completely ignore WoW arena, or any kind of innovation at all with WoW, only releasing uninspired expansions to further milk addicts. Now players have gotten bored enough to jump ship to FFxiv and they are losing their cash cow. Lets not forget the entirely forgettable Warcraft movie too.
Even Overwatch was supposed to kill cs:go but it is basically treading water now, especially since the sequel was delayed and Jeff Kaplan has left. The pro league is a joke compared to the big names. HOTS was abruptly abandoned. Hearthstone was basically their only great success (but I assume is stagnating like everything else now), without it they basically have nothing which is why they have turned to remakes recently (and have failed spectacularly with wc3r and had to outsource d2r in response). They basically have nothing substantial until d4 or Overwatch 2, both of which were delayed until 2023 at the earliest. Now with all the workplace culture scandals they are hemorrhaging talent on top of everything and have effectively lost their position as one of the most desirable studios to work for in the industry.
I don't think Blizzard knows how to make good games anymore, that part died long ago.