First up, I worked for Valve for 7 years, so I know this stuff.
Second, no, Steam has lots of smaller games, but Battle.net has the whales. Even beyond Battle.net, there are other enormous services out there that eclipse steam. Steam, while in my opinion the greatest thing to happen to all gaming period, does not have World of Warcraft. Steam does not have League of Legends. Steam does not have Honor of Kings. Quite literally, all the biggest PC games are not on Steam.
Third, I have been hearing from friends all day long about who was toast and this is something that isn't being reported -- the vast, vast majority of the layoffs are coming from Treyarch, Sledgehammer, and Raven, along with a huge portion of their Xbox team responsible for physical distribution. In other words, their COD studios, which aligns with exactly what I said. They didn't buy Activision for COD, and it wasn't redundancies because Acti-blizz largely does not have physical distribution teams.