I agree if Cavil really is gone, than consider myself one of the ones no longer really interested in the DCEU.
Much to the same case for me when Chris Evans finally hangs up the Captain America Shield after the Infinity Wars because that was my dude within the entire MCU and the only real reason why I continued watching the Marvel movies.
It is sad that Cavil in Justice League was to me the best incarnation of Superman he had, but as you said, his screentime was next to nothing. This felt like the Superman that I know and have the 'good feels" about.
Even though Justice League wasn’t great, I was really looking forward to the potential of Affleck, Cavil, Gadot, Mamoa.
Personally, I want to see Affleck and Cavil in one Really Good Super Hero movie, just one.
They deserve better than this, and I’m really saddened that if all of this is true, than my interest for the DCEU will have taken a decline.