Especially GTK 3, which was incompatible (by default) with anything that didn't resemble the imagined wayland future, including non RGBA8888 displays or systems with different GL and X11 visuals.
Every X11 app is already composited and scaled by compositor and has top-level X window where it could set any property it wishes.
Also, unless you're running Xsgi, it's not a given that every app is scaled by compositor.
Honestly, the right way was always to not assume any given DPI, and instead have every top-level window be DPI-aware and have the toolkit update that information dynamically.[1] Or optionally go all-in on vector dpi-independent approach.
[1] The only correct way to handle hi-dpi on Win32 GDI, where scaling is broken partly by old GDI assumption that DPI didn't change, and by applications that hardcoded 96 dpi