I don't think libsystemd is a particular problem. Or at least it being linked in only made the job of writing the exploit slightly easier: there's enough services running as root that will pull in a dependency like this that the compromise still exists, it just requires a few more hoops to jump through. And systemd has in fact deliberately made the notification process simple specifically so people can avoid the dependency (if not for security, then simply for ease of building in a way which supports systemd notification but doesn't need anything else).
Dependencies are a liability, for sure, but I think a lot of the reaction there is not entirely helpful. At least, the size of the dependency tree in a package manager is only about as good a proxy for the risk as number of lines of code is for software project progress. Dependencies need to be considered, but not just minimised out of hand. There are plenty of risks on the reimplement-it-yourself side. The main thing to consider is how many people and who you are depending on, and who's keeping an eye on them. The latter part is something which is really lacking: the most obvious thing about these OSS vulnerabilites is that basically no-one is really auditing code at all, and if people are, they are not sharing the results. It should in principle be possible to apply the advantages of open-source to that as well, but it's real hard to set up the incentives to do it (anyone starting needs to do a lot to make it worthwhile).