I don’t understand this logic. Can you explain why having code in more than one place is a bad thing?

For a simple script that does one thing, it's overkill.

Yes. So you keep that simple script simple, and you let systemd do the heavy lifting.

But systemd doesn't do any of the above listed things automatically... You'd need to write an entire script for sytstemd to take advantage of it. An entire script you could just have run with cron on boot.