According to one of Polygon's anonymous sources with knowledge of the game's status; as Black Ops 4's release date got nearer, it became evident that developer Treyarch would not be able to complete a single-player campaign in time. While each of the games contains a dedicated campaign mode, most players skip it entirely and jump straight into multiplayer.
If this information is correct, it would not be a surprise.
We're not suggesting that Activision would ship a Battle Royale mode so bad that Rock Paper Shotgun notes, "If Radical Heights was any more early access, we might well be trying to play a design document". Even so, developers didn't ditch the single player campaigns completely.
Perhaps a precedent closer to home for Activision is Overwatch. And yes, it's obscenely successful thanks to sustained monetisation via cosmetic micro-transactions. "Paying the same price for what many will perceive as a smaller game may turn into a tough sell for the Call of Duty team". The tagline for the upcoming game is "forget what you know", which has a certain sense of irony as the longest-running Call of Duty subfranchise looks to chart new territory. Numerous job listings on Beenox's site states that the studio is "currently dedicated to the development of the Call of Duty franchise." .
Call of Duty is not the only franchise jumping on the Battle Royal bandwagon.
Call of Duty and its multiplayer mode are practically inseparable.
Call of Duty could be getting a Battle Royale mode with Black Ops IIII. It's unclear if this is something separate from the expected zombies mode and multiplayer but it wouldn't be unexplored territory for the series.