100yo Coastal Oak. Seems dead. When do I call it?

Five months ago the tree turned brown, most leaves fell off. There were some issues with a buried root collar and drainage around the base, got waterlogged. I've corrected the problems, done some fungus abatement, fertilized with 20-0-6. Some trunk rot, a few borer holes. cross-sections of some small branches low in the canopy show a little green under the bark, but the few green leaves closest to the trunk have all disappeared or turned brown at this point.

Brought in two arborists. Each one said it's a 50/50 shot that it comes back ... but also recommended treatment programs in the four figures, so I'm still skeptical.

I guess what I really want to know is how brown&dead&dormant can these get and actually rebound? How would I know? How long should I wait? I'd love for somebody with experience and no commercial interest to opine on this. Is there hope? Or is it time for chainsaw rodeo?

Much thanks.

Here's a picture:

https://imgur.com/a/Owg8j08