The issue is POSIX standardizing legacy stuff like shells, thereby tempting people to write "portable" software, leading these technologies to ossify and stick with us for half a century and counting. Someone comes along and builds something better but gets threatened for not following "the UNIX way".
This is a very good point. I wonder how hard it would be to get POSIX to standardise a scripting language that isn't awful.
Probably never going to happen. There is a dearth of good scripting languages, and I would imagine any POSIX committee is like 98% greybeard naysayers who think 70s Unix was the pinnacle of computing.
POSIX does not specify the init/rc script system, so it's not a factor here at all. A POSIX-compliant system could use Python scripts. macOS (which is UNIX 03 certified) uses launchd. A POSIX system has to ship the shell, not use it.
And FreeBSD isn't actually POSIX-certified anyway!
The real consideration here is simply that there are tons of existing rc scripts for BSDs, and switching them all would be a large task.
And cesarb is correct - the issue isn't scripts; it's shell scripts, especially Bash and similar. Something like Deno/Typescript would be a decent option for example. Nushell is probably acceptable.
Even Python - while a terrible choice - is a better option than shell scripts.