Blizzard has many games in their pipeline, both live getting future updates and expansions, plus games under development.
Blizzard also makes and has a shit ton of money, even more if we count ABK and MS. So they can cancel a game with no problem even if spent 6 years on it. The thing is that they spent 6 years on a Minecraft + battle royale clone and still were several years away from releasing it. So maybe the project was a total mess and was better to cancel it and put the money instead in something safer like Star Craft 3.
Blizzard has many super succcessful projects alive: WoW, Wow Classic, Heartstone, Diablo IV, Overwatch 2, Heroes of Storm, more refurbished old, Diablo, Starcraft and WoW stuff, plus mobile games. The next big Blizzard game should be Starcraft 3. Or maybe even Warcraft 4/5.
Regarding Activision, other than CoD and in a way smaller scale Spyro and Crash they also have other ones like Tony Hawk, Guitar Hero, Skylanders, Hexen, Heretic, Pitfall or the Sierra games to name a few. I think Tony Hawk is the most likely to be brought back in a new game outside remasters/collections, followed by Guitar Hero and Skylanders. I think it's unlikely, but being Indy has a first person camera, maybe MS could resurrect Pitfall to make their own Uncharted.
Heretic and its sequel Hexen were made by Raven and published by ID Software. Now are both under the MS umbrella. Maybe MS could ask them to codevelop a GaaS third game of the series.