Yup - it turns out pulling out the core of your graphics in a graphics programme can take a lot of time to get right (certainly took years in my own small project).

Who knows, maybe GIMP will jump straight to Gtk4.

There's a good roundup of happenings in 2017 in GIMP here:

https://www.gimp.org/news/2017/12/31/gimp-and-gegl-in-2017/

[EDIT] There are links in the above doc to sponsor GEGL developers [/EDIT]

It's encouraging that GEGL is seeing use in other projects and receiving patches, maybe the GEGLisation isn't too far from completion.