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.
Call me a noob but why should this problem exist even today ? Why are APIs such grossly incompatible with previous gens such that a complete overwrite is necessary?
Well i think GNOME and even GTK has very few contributors and mainly sponsored by RH and other companies. I suspect they are often getting paid to develop features for subcontracting and other times they want to "modernize" the toolkit and they just write a new API with additional features. We have seen this during DBus introduction, GSettings, Clutter etc... etc... Plus GTK+ is C plus lots of boil plate code to make it "object oriented" which turns to be even a bigger pain when pure GTK+ apps need to update their APIs to catch up with the latest.
But imo this is a general trend on Linux except for the Kernel where Linus and co do a great job to prevent API breakage. The others not so much.
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.