The thing I can't understand is that netplan is just a bunch of janky C that is almost entirely parsing logic and the thinnest wrapper humanly possible over sys

The thing I can't understand is that netplan is just a bunch of janky C that is almost entirely parsing logic and the thinnest wrapper humanly possible over systemd-networkd/networkmanager that does all the real work. And the abstraction leaks for anything but the simplest cases so your files are targeting a specific backend anyway. systemd-networkd already gives you simple declarative INI networking files and a dbus interface. I can't find any value-add over just using its backends directly.