I know I am being a bit bitter for an open source project with less funding, but GIMP was supposed to be a poster boy GTK application in the first place.
Anyway, turns out maintaining a professional image editing program is a lot of work. While it is true many fanboys named gimp as some kind of foss and gtk showcase its not like most of them were actively contributing to either gtk or gimp.
There was never any longterm large scale commercial investment in gimp similar to how redhat threw massive resources into gnome and gtk.
This is wrong. GNOME was already well underway when Qt switched to GPL. Before that it used QPL and before that yet another license.
"Sadly, the KDE effort is based on the Qt user interface toolkit which is a non-free library. The main problem is that modification of the toolkit is not allowed. The ability to modify the interface toolkit is a requirement of free software[6] and of the Open Source Software Guidelines[17]. Although Qt is released in source form, permission to redistribute changes made to it is denied."
At the same time there is an ongoing transition to use GEGL as the pixel manipulation API, which eventually plugins also need to be updated for.
2.10 is the release which provides all the new APIs for plugins to use, so they can start updating (but existing code will work). 3.0 will then remove the old pixel manipulation APIs, and also transition to GTK3+.
This hopefully keeps the burden on plugin devs to a minimum, and keep the compatibility clear for users (plugins for GIMP2 dont work on GIMP3, and vica versa).
This is a serious question in general. There will always be s steady stream of significant upgrades/changes of any popular open source project as later developers add in things they consider important. At some point the decision has to be made to stop following those changes. This decision is usually made by default when it turns out that no one can be bothered to follow the changes of a project from another project.