Rather than API/ABI stability, I think the problem is the lack of coherence and too many fragile dependencies. Like, why should a component as essential as Systemd have to depend on a non-essential service called d-bus? Which in turn depends on an XML parser lib named libexpat. Just d-bus and libexpat combined takes a few megabytes. Last time I checked, the entire NT kernel, and probably the Linux kernel image as well, has no more than single-digit MBs in size. And by the way, Systemd itself doesn’t use XML for configurations. It has an INI style configuration format.

that's why they are doing varlink now