A large part of even non-trivial applications do not do custom drawing - and many do not do any custom drawing at all. In the cases where this is needed, you only need to fix the particular drawing code and nothing else.
> the porting effort to newer versions of GTK is also just a few lines of code, if even that
Even if that is the case, it still has all the other backwards compatibility issues i mentioned.
> Well, GNOME will probably not make this effort, as they dropped GTK1 around twenty years ago.
The effort i mentioned here was on designing backwards compatible APIs and libraries, not about supporting something specific to Gtk1. I do not expect GNOME / Gtk developers to support Gtk3 for long, let alone Gtk1.
> Again there is nothing to take into account here.
The "that" in "have to take that into account" was backwards compatibility, not DPI scaling.
Look, we're going in circles and i do not feel like having to explain things that i believe should be self-explanatory all the time. So there is no really any point continuing that discussion, it is not like i expect any Gtk developer to suddenly come across my posts and somehow get enlightened or something and Gtk stops breaking itself (and well, even if one does, they'd have a hard time convincing the other developers, so it is probably better for their psyche if they do not understand what i write :-P).