Switched to Brave recently over this.
You can try to keep the code, but every upstream code change becomes more painful to integrate. It is going to cost more and more, requiring more and more expertise with the Chromium codebase, like it is some kind of technical debt. Not even counting the potential security issues you can introduce by keeping the code alive, with Brave being a target that is large enough to matter for hackers but perhaps too small for security researchers.
The money has to come from somewhere.
All other browsers have some kind of reasonable story behind the investment made into their existence (yes, even Chrome). Brave doesn't have any, and if the crypto ads stopped paying the bills, there must be something else that comes around to do so. Frankly, I wouldn't trust anyone who's put in that sort of situation with a piece of software I use to access anything important. Doubly so if crypto is a load-bearing component of the current status quo.