That isn’t entirely true. You can change it as long as it is done via package overrides or overlays. Sure it rules out just compiling/installing something into your root unless you package it first but you can change it.

I honestly like the fact that it effectively enforces every file in the immutable parts of the OS to be traceable back to some package.

Silverblue and other distros like it fix this by not changing the running system. The pending update just becomes the running system on next boot.