This is possibly a very unpopular opinion, but I think Treyarch using the IW engine is not just okay, but actually a great thing for COD zombies. Now, the first thing that comes to mind is probably the loss of uniqueness between Modern Warfare and Black Ops games in terms of art style, but Treyarch has pretty much always used a heavily modified IW engine. And if Black Ops 7 is any indication, good atmosphere and design can make them feel very different.
The democratization of attention to detail across titles is also a net positive I think. Remember those videos comparing Cold War to MW2019 and Cold War looking like a mobile game in comparison (empty weapons having bullets during inspection, no blood/gore decals on zombies, less dynamic sound design, etc). Now the all the games can have good attention to detail instead of one being wildly better than another.
Also, staged reloads is nice for zombies, and has saved my life a ton since I don't have to restart the full animation after receiving a teammate, sprinting, meleeing, etc.
Let me know what you think about this!