this is problem #1 of Linux Desktop. Toolkits and Libraries constantly changing and reinventing the wheel. Actually a big part of developing an GTK App is actually catching up with the API changes. As a proof of this see the early how-tos of GTK 3.0 and 3.2 and see the recent ones and see how much it changed.

There were plenty of apps changing from GTK to Qt awhile ago i suspect GTK4 will be the next. As for Gimp and Inkscape it will probably take another decade to catch up.