XLEngine vs Daggerfall Unity is the exact example I had in mind.One example I can think of is to compare the two Daggerfall engine replacement projects I'm aware of: the XL Engine and Daggerfall Unity. XL Engine is currently closed source with the developer stating that he will open source it when it is ready. That was two years ago, the dev went silent around July last year and there appears to be no indication of when the next update will be available. Conversely, the Daggerfall Unity project was open source from the start and has seen a lot of development in the short amount of time since it started, multiple programmers are working together to get it finished and has an active modding community too.
This only confirms that while it's been open-sourced, not many people even know about it. He should have made it public from the start.XL engine did get open sourced if I understand correctly: https://github.com/luciusDXL/XL-Engine
Unfortunately it was just too little too late. He lost the PR momentum he had and no new devs joined up, from what I understand.
I'm sad for the XLEngine's fall because Unity is proprietary and so Daggerfall Unity isn't strictly speaking a free software