Look at the state of WordPress: the (B?)DFL actively bans people from the community for critiquing his self-described "nuclear war" waged against his biggest competitor in the hosting space, which "nuclear war" has caught thousands of members of the community in the crossfire. And yet we see no fork. Why? Because forking is hard and fragments the community, so people would rather put up with a tyrant than deal with the risk of instability. This is no different than tyrants in any other environment.
If a project has good governance established from the beginning, including a reasonably democratic process for contributors to elect the executive function, then the community can be reasonably sure that they won't feel the need to fork in the future because they have recourse if things go sour.