Suppose I want to write my own competitor to systemd-resolvd. I want it to work with systemd-networkd, in the same way as systemd-resolvd does. Or just patch dnsmasq or unbound to be able to do that. Is there a stable interface for that? An RFC I can read? Any guarantee they won't change the interface and break my version, while they update both of theirs together?
Judging from the manpage and a quick look at the code, the only thing for which systemd-networkd and systemd-resolved interact is to get the DNS servers pushed over DHCP, which it does over the D-Bus interface that falls under the interface stability promise[1].
That is not the only thing for which they interact, see e.g. systemd.network(5). The systemd-resolved.service itself reads the systemd.network config files. This is what people mean by tightly coupled. But more importantly, you're telling me what it does currently, not what it's going to do tomorrow.
Suppose it did currently only interact through a stable D-Bus interface. What stops them from adding some new interaction and supplementing or replacing the existing one with it? You're left with a guarantee that systemd-resolved will continue to support the old API that systemd-networkd may stop using.
