I find the "monolithic, incomprehensible, unintrospectable" complaint particularly odd. It has made it so much easier to slice and dice logs, make sure you have logs in almost any conditions, massively increase the ability to request information about running processes, and how they are related etc. "systemctl status [service]" alone is sufficient justification for system. Being able to get resource usage
per service instead of just per process with "systemd-cgtop" too. "systemd-analyze" is also fantastic (try "systemd-analyze time" and "systemd-analyze blame" and "systemd-analyze critical-chain"). Systemd have made it vastly easier to learn about the boot process without being forced into intrumenting dozens of shell scripts.
As for simple incantations, "sudo service nginx restart" works just fine on Debian with systemd. It's trivial to provide wrappers. But not exactly hard to learn "sudo systemctl restart nginx" as an alternative. There is a small but real learning curve to replace your old habits, yes.
As for "sudo killall nginx" it is indeed easy, but it's also an awful habit. Suddenly you're on a server with multiple different nginx instances (in different containers, perhaps; been there, killed those by accident; stopped using killall). Or you're applying killall to a process that will leave a mess. Or you have a server process that spawns children that double-fork and won't die just because you've killed the server.
That simple case is indeed simple. Until it isn't. And that's one of the many caveats where systemd goes far further than most of the alternatives by e.g. using cgroups to keep track of related processes even when they do stupid stuff to confuse you. In return you get to do "sudo systemctl stop [unit]" and know that not only will it be stopped properly - along with all related processes, but any other dependencies will get shut down cleanly.
We do have choice, but in most cases where the "dead simple" cases matter, it is a matter of relearning a handful of commands (with wrappers for some helpfully provided by some distros) for the dead simple cases to work, with the bonus that all kinds of ugly corner cases most people don't even think about just stops being a concern.
If your needs are dead simple, then systemd is dead simple too. When your needs are not dead simple, most of the alternatives can't solve them properly, and you often end up having to cobble together awful monstrosities. Or trying to figure out what in the world went wrong.