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.

IIRC main Fedora used to not do this until some update crashed people’s sessions including the update process which left their install in an unbootable state.

The ostree based versions like Silverblue avoid this by their updates not touching the running system and instead creating a new folder structure with the updates applied that will be booted into on next boot.