It's exactly for "old broken apps". What I generally do on my Gnome systems running 4k screens is to use 125% or 150% scaling, and then set larger default font

It's exactly for "old broken apps". What I generally do on my Gnome systems running 4k screens is to use 125% or 150% scaling, and then set larger default fonts for the system. Fractional scaling helps keep old apps legible even if imperfect, but updated apps which respect system font size render great (Gtk+ widgets auto-scale to fit the text). Unfortunately, this approach ain't gonna work with Wayland as well.