Portability matters.

In general: Yes. In the specific case of an init system for a specific OS: Not so much.

This is nicely demonstrated by none of the non-Linux OSes embracing any of the options you listed. They all want something that plays to the strength of their specific systems over some generic Unix thing.

If using Linux specific features was the only approach to security, I wonder how OpenBSD exists.

It is the best approach we have on anything running a Linux kernel.

systemd cannot compete with their simplicity, maintainability, smaller attack surface, and the list goes on and on and on.

It is also easy to have really simple code that does nothing interesting whatsoever. And for something that does not do much at all, the fork-dance that e.g. s6 does is pretty complex.

Maintainability also does not seem to be a big issue for systemd at this point in time either.

The smaller attack surface is relative as well: systemd-the-init is a bit bigger than the ones you list. But the difference is not as big as you make it sound and an init system does not do many interesting things that can get attacked by either.

On the other hand systemd can seriously lock down any service it starts (and does so out of the box for anything from the systemd project and many upstream projects that ship locked down systemd unit files). The init systems you listed do can not do that directly and either need helpers (which increases their attack surface again) or just do not bother. Considering that a init system starts way more lines of code that do more security critical things than an init system: I think this lockdown does lead to a smaller attack surface of the system overall.

systemd de-facto causing other projects to put in (sometimes radically) more work than they should have to, is not okay;

Somebody has to invest work to make things convenient and easy to use. You either run with what everybody else uses and share the effort or you do not and do the work all by yourself.

This is in no way systemd specific.