It's a graphic design program. If there is a project where tight integration between the graphic library and the rest of the codebase is expected (for performance reasons) and hard to avoid is here.

In wireshark, for example, its meat is in the networking code, and the important skills of the developers are there, on inkscape, on the other side, the non-trivial contributions probably happen very near to the graphic frontend. If GTK goes to the gutter, retraining the developers to get to the same level in Qt will set Inkscape back years.