Linux

To set the record straight, since you apparently have no idea of the history: systemd isn't the original Linux init system, and wasn't foisted on the Linux community because it was technically superior for most people's use cases. It still isn't the only viable Linux init system, but it pulled a Microsoftian embrace-extend-extinguish on udev, which makes it more difficult to switch away. Its current popularity is still not based on technical merits. Instead, it's political, because most people don't care about what init they're using and most distro-makers take the path of least resistance.

It's true that you're not required to use all of the individual executables that comprise systemd, but most distros will require you to install them. So they're still present as unwanted clutter, and bugs could still pose a security risk if an attacker can run the executables. (This doesn't mean that OpenRC or runit would necessarily be any more secure—every non-trivial piece of software has bugs, and some percentage of those are going to be security-relevant. You're not required to care about small amounts of on-disk clutter, either, but some people choose to make their system partitions small and micromanage the contents even if they're not working on embedded.)

Compiling your own copy of systemd without the clutter, judging from the contents of the systemd ebuild, requires setting more than 30 compiler options. And then installing the result manually without trashing your system. Not trivial, in other words.

If systemd works for you, then by all means use it, but accept that other people may choose to install something different on their own machines for what you consider to be bad reasons, or no reason at all, and arguing about it just annoys them without providing any benefit to you.