Even if GTK2 were to get hidpi support, it would still require making the same types of changes in the applications to support them that would be required to po

Even if GTK2 were to get hidpi support, it would still require making the same types of changes in the applications to support them that would be required to port it to GTK3 and get hidpi there anyway. Only the most trivial applications without any custom drawing/widgets would have hidpi working without any changes. Rule of thumb, if the app's code uses raw pixel units anywhere, that would need to be modified to account for scale. You would probably need to modify all the GTK2 theme engines too.