Matt controls the WordPress foundation, owns and operates wordpress.org, is CEO of Automattic, and votes 84% of its stock (https://news.ycombinator.com/item?id=42657150). AFAICS, there's no one who has the means to remove him.
There's always ways to remove someone completely gone off the rocker. Employees or customers voting with their feet, some big and established name(s) forking off the project, getting the target arrested (if only by provoking an escalation), getting a judge to declare the target mentally unfit, and various illegal methods as well.