To summarize:

stickynotememo: "How would you install packages (or update the kernel)?"

You: «You'd use sudo or su (of course) but I'm going to distract you from that with an advertisement for the work I'm doing on my personal project!» [0]

But, perhaps I misunderstood what you wrote. So... I'll ask stickynotememo's question in a different way:

  How would a user of your system update the Systemd collection of software to apply a critical security update?

If the answer is something like "I'd rebuild the read-only root partition with the new code.", then I ask:

  1) How does the data in that root partition get rebuilt? If it's on another computer, how does one control access to the root-partition-rebuild computer?
  
  2) How does one instruct the user's computer to use that newly-rebuilt root partition? How does one control access to the system that permits one to load a new root partition and/or change which root partition to use?

  3) How does a user recover when an update fails or is faulty in a way that your smoke tests didn't catch?

Please keep your answers concise.

[0] Personal projects are great, and I strongly encourage them. However, the way you've answered is what a greybeard would describe as "pulling a fast one". Pulling a fast one does not earn trust.