This is indeed practical advice on how to do a transition, but my answer was primarily in response to the OP’s question, which was about the reasons why people don’t even try to transition. I don’t at all suggest that a slow transition is in any way invalid. But to even get to that, the reasons for not transitioning have to be overcome. And for any sizable project, community cohesion is going to come first, or else the result will be less than the sum of its parts.