You've never written portable software, have you? You can't write portable software without a common interface. Have you ever looked at what autotools vomits out? You'll see shell scripts containing the likes of "echo $1 | sed 's/^x//'" in order to just get things to where they have a chance of sharing enough commonality to compile. Imagine how much worse it would be without POSIX. Nothing would run on anything that the developers didn't account for, this includes OS (plus version), environment, and hardware architecture. That is the world you are asking for, not the one we presently live in.
> What is the benefit to sticking with an rc script architecture designed to run a handful of processes on a pdp?
Freedom, for not only the end user but all the way up to distro packager. Systemd is designed to be non-portable, and force a network-effect pressure. I was similarly suspicious about WSL designing toward a linux API instead of POSIX... an effort to reduce the potential for alternatives emerging in the future, where your choices are windows or linux.