I’d love to see a case study of how Blender managed its massive 2.8 UX overhaul. It’s the only major open-source application I know of that’s pulled something like that off.
2.49 was the last one with the really inscrutable old UI, 2.5 was a big overhaul but still had a lot of basic functionality buried behind hotkeys or menus, and 2.8 is the relatively recent one with a lot of that basic functionality exposed.
I don't follow the blender development process too closely, but I think Blender Guru's videos kicked off a lot of the 2.8 design work: https://www.youtube.com/watch?v=UWacQrEcMHk
From my extremely limited involvement in the community it seems like it was started by one artist publishing a very dreamy vision of "this is what the future of Blender could look like" to his followers. It seems like people liked it enough that a lot of it was eventually incorporated. It might have also started a wave of enthusiasm for more people to become involved.
[edit: changed 1.8 to 2.8]