I'm not going to discuss the merits of portability or what "typical developers use". I'm questioning the idea that systemd is somehow more portable than its alternatives. That belief can only be rooted in plain ignorance of the actual situation. It has absolutely nothing to do with what typical developers use, and to that end your comment is a pointless derailment.

What I mean is that there are init systems that only require POSIX compliant APIs and as such are portable across systems that implement POSIX, e.g. GNU/Linux, BSD, Solaris, AIX...

systemd requires (in addition to POSIX) Linux APIs and is therefore not particularly portable by any reasonable definition of the word. You can use it if you are running a Linux kernel, and that's it.

> What I mean is that there are init systems that only require POSIX compliant APIs

Those are irrelevant as they can't even do basic tasks like shutting down the computer or restarting it. Feel free to find a POSIX API for Linux' reboot syscall.

Some systems also require mounting additional filesystems, again not possible in a world limited to POSIX interfaces.

Sandboxing, SELinux, ...? Not in POSIX.

And so on.