It's not a similar argument at all. I can write a unit file without breaking systemd. If you can't, then the problem isn't with systemd, because I'm a veritable suckfest at my job. So if you're routinely breaking systemd, I'd look elsewhere for the source of the problem. Also, systemd is actually entirely modular. So if you build it from scratch, to a good extent you get to choose what you want to integrate. Arch's systemd is way different from Debian's, for example. Hell, it's even more advanced than Fedora's.
The difference is in the tools you use to do the job. My grandad's hatred of metric was based entirely on the fact that the Germans and Japanese used metric fasteners, and that he had to buy new tools to use the job - his AF were less used, and his Whitworth were almost entirely obsolete by the 80s. People's hatred of systemd is that Poettering is an author, Red Hat have championed it, they have to learn something new and their knowledge is becoming obsolete.
If some people had their way we'd be using a bicycle powered smoke signal generator instead of a phone.
(As an aside, plastic fastenings in engines are extremely uncommon, as the heat and vibration in an engine will destroy them fairly quickly. If plastics or resins are used, it's usually as an adjunct to metal, or in components where it's necessary for strength and security to replace the fastener after it's been taken apart. Your dad was, in a sense, right about things being glued or riveted more these days, but that's largely because the ability to service complicated parts that started appearing in cars in the 80s and 90s has meant that even experienced mechanics don't have the tools to work on them - the dual clutch solenoid units spring immediately to mind. Nevertheless, the source code for systemd is available for you to hack away at all you want.)