Sure but even Vivaldi has a few C++ folks on the dev team, how many more is it to maintain an existing API to changes in the codebase, especially when any of the hard parts of that work only need be done once among all of the forks.

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.