Even in the “golden age” of COD, zombies was THE thing that sold post launch DLC.
Look at the trailer for Map Pack 3 that Treyarch released in the WaW days. It’s a map pack with Der Riese and 3 multiplayer maps, yet 80% of the trailer is dedicated to showing off Der Riese. That was literally why people bought and played WaW back in the day.
Even as someone that was actively playing zombies during the WaW days, it’s still kind of shocking to me how popular it was relative to everything else. It was so popular it was almost exclusively used to sell post launch DLC.
No “main quest”, no convoluted story, just loading in and trying to get the highest round. I promise you absolutely nobody back then found it boring or “lacking content”. There’s so much retconning from modern zombies players that the old modes were boring, but even in the golden age of COD it absolutely carried post launch sales.