The threat model is a malicious entity has limited access and can swap out the intended script for a malicious one, and have it run in a root context.

This is equivalent to not having signatures on your repository packages and saying "no biggie, we rely on transport encryption". Might work in most cases, but there's a reason good security uses layers. A failure at any point-- TLS downgrade attack, repo compromise, proxy compromise, DNS poisoning-- can result in your preflight script executing malicious code.

Requiring code signing with a pinned cert would solve this issue, but would be terribly out of character for the company that brought us a hidden local REST API to bypass OSX security prompts.