Arguing that the advantage of systemd is portability is rather bold!
And even if portability was the point I'm not sure I see the big deal. Writing an RC script from scratch if the software you use doesn't provide it is generally trivial. The vast majority of the time you wouldn't have to do that anyway as it ships with your OS's packages anyway. Sure, systemd might be "tidier" with its standard APIs and whatnot but it's also a lot more complicated and opaque than a bunch of shell scripts running one after an other. And if you're serious about sysadmin you'll have to learn shell scripting one day or the other anyway.
>Init scripts were always a poor-quality mess, non-portable among systems, non-consistent, non-deterministic.
They're non-portable, that much is true but so is systemd, that's a weak argument. If everybody had adopted FreeBSD-style init it would be equally as portable, it's a self-fulfilling prophecy. With that logic we should just all ditch un*x and start running Windows since most people already use that anyway.
The rest is nonsense. It's poor quality, non-consistent and non-deterministic if you write them that way. Sure, shell scripts being turing-complete opens the door to a lot of nonsense if people go wild and gives more latitude for very sloppy code but it doesn't have to be that way.
>the people who sit down and actually write the code we all use find the services systemd provides valuable
Systemd has been pushed down everybody's throat for a while now, saying retroactively that people use it because they find it valuable is a bit of a stretch. I'm sure many of them use it because that's what's available. I wrote a bunch of systemd unit files myself, I assure you that it wasn't meant as an endorsement.
Besides it's only one side of the equation. Maybe it's nicer for the people writing the unit files, doesn't mean that it's a good thing for people actually having to use them. I'm sure many software maintainers would prefer if everybody ran the same OS on the same hardware with the same use cases but that's not how the real world works.