> You'd just put in a change hook like you'd restart a service after changing its config file.
It's not the same. While servers restart on reboot, the config file is not automatically compiled. Also if you don't reboot, most servers are restarted through a uniform interface. Dconf makes an exception without need. You need the dconf infrastructure at deploy time (it might be an installation), and you need to know how to use it.
I've wasted hours figuring out how the configuration is to be used. In the end I still had terrible bugs. I'm glad other programs just do the parsing for me. It works, there never was a problem to begin with.
> The current GTK theme or scaling factor is read all the time during rendering.
I think you are missing the possibility of parsing a configuration once at startup and storing the result in memory in suitable datastructures. It's not like conventional programs would parse the configuration over and over. Fixed binary layouts are very bad for interoperation.
Btw, there is no memory usage problem compared to an mmap solution if every program just parses the _relevant_ information. And it also doesn't preclude a live-update mechanism -- this just has to be done with care. Not every information is trivially live-updatable, as illustrated by the breakage I mentioned.
> I can just set them using "gsettings set org.gnome.desktop.interface.scaling-factor 2"
This can be also done with text configuration files. Some programs do it. Most do it not (including dconf, which doesn't update your original input file) because the tradeoffs are bad. It has nothing to do with the question whether or not the serialization format is text.
Text vs binary is just this question: Do I need absolute performance or absolute discoverability? In the case of configuration files, the answer is clear.