Certainly losing his technical talent is likely to have contributed to the problems, but I suspect it's 100x more of Chrome's dominance and Mozilla's reliance on Google's search engine contracts for revenue.
Maybe he's just not a great guy all around.
https://nulltx.com/brave-browser-draws-criticism-over-lack-o...
> Publishers must verify ownership of their properties with Brave in order to receive contributions from Brave users. If a publisher has not verified ownership, then a user’s contributions will be held in reserve inside the browser for 90 days. The browser routinely updates an internal list of all verified publishers to determine whether a property can receive contributions. At the end of the 90 day period, any contributions marked for unverified publishers will be released back to the wallet. No funds leave the browser except to go to verified creators.