This is a non-issue. The `!#/bin/bash` at the top of scripts will tell the interpreter which shell to run. And any one liner I've found online for bash has also worked on zsh (for me), but even if it didn't: `bash -c ls`

Yes exactly.

Today I learned people actually write / use scripts without specifying the interpreter at the top of the script. This is like writing a Ruby script and expecting it to work when calling it with the Python interpreter.

Yeah, it's actually not installed everywhere (e.g. FreeBSD).

Anywhere that matters. Besides, still more places than fish or zsh.