It will 100% affect the COD Fanbase. Not then young ones like 10-16 year olds, but the a bit older ones grew up with call of duty and many still have a soft stop, but it just got a bit stale. Battle Royale + COD honestly sounds amazing. Only Battle Royale + Battlefield sounds better.

Cod got the best close quarters gunplay. That in a battle Royale game sounds epic. Battlefield has the best big battles and vehicle gameplay and long range weapon feel which would make it also amazing.

Fortnite took away from PUBG so why wouldn't cod take away from Fortnite. Many people don't love the building because of building, but because it ensures fast paced gameplay. Heck I would also say lots of people don't necessarily think the building is a great reason to not switch, it is just there neither good or bad overall, it is just that PUBG really is kinda shit in comparison.