I wonder what will Vivaldi do. They say that their built-in content blocker is "good enough" that you supposedly don't need uBO (I very much disagree) but they also keep MV2 extensions working to this day.
Their tech stack is heavily JavaScript-focused, as their entire UI is written in JavaScript.
As a counter example, Brave is heavily invested in C++ and Rust, and I believe they could handle that work much better.
Even if they don't want to handle it directly, this is the kind of thing a single sponsor can pay Igalia for, who have shown the ability to make entire new Chromium subsystems like MathML. There is no shortage of C++ browser developers in the world to do maintenance work.
[1] https://vivaldi.com/blog/manifest-v3-update-vivaldi-is-futur...