The list above is incomplete, there are also the logo (with the currently controversial terms of use), the website, social network accounts, the mailing list and all related infrastructure. You yourself admitted it is becoming increasingly difficult to pick people who are not affiliated with The C++ Alliance to fill important roles, such as review managers and reviewers. Given that you alone are funding the Alliance, I'd say you have a pretty strong leverage on it, and by extension on Boost.
In these examples, the C++ Alliance is not taking control of anything. It controls the things it creates and maintains so they can continue to exist. And even make things open so other people can continue without them if they ever want to. If person A is providing X, even if you share everything you have done so far, you will always have control over whether you can keep providing X. That's not person A having control over something that exists. It's person A having control over something that might exist. If even this kind of "control" (more like self-determination) is to be avoided, the only solution is for everyone to be prohibited from providing anything because they might one day stop providing it. Unless the point is not about control and more about not wanting to become dependent on person A. But in that case, the solution is to actively find other people who will contribute. The solution is not to stop people from contributing just to avoid a scenario where they might stop contributing one day.